java中的泛型类及其使用

•  泛型的使用

• 集合的使用

　　一般集合的使用方式是：

　　比如有一个Person类

package com.atguigu.java;

public class Person {

//    @Override
//    public boolean equals(Object obj) {
//        return false;
//    }
    
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }

    public Person() {
        // TODO Auto-generated constructor stub
    }
    
    public Person(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    
//    private static int init = 0;
    
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
        
//        return init++;
    }
    
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person other = (Person) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
    
    
}

　　然后定义一个Person集合

package com.atguigu.java;

import java.util.ArrayList;
import java.util.List;

public class TestGerner {

    /**
     * 对于集合没有泛型的情况:
     * 1. 放入集合中的对象可以是任意类型. 
     * 2. 获取元素后, 需进行类型的强制转换. 
     *    为了类型转化是安全的，可能还需要instenceOf判断
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        List persons = new ArrayList();
        persons.add(new Person("AA",12));
        persons.add(new Person("BB",12));
        persons.add(new Person("CC",12));
        persons.add("string")
        
        Object obj = persons.get(0);
        
        Person person = (Person)persons.get(0);
        
    }

}

　　 对于集合没有泛型的情况:
  　　　　* 1. 放入集合中的对象可以是任意类型.
 　　　　 * 2. 获取元素后, 需进行类型的强制转换.

 　  如何把一个 list(集合)  中的内容限制为一个特定的数据类型呢？这就是 generics 背后的核心思想。这是上面程序片断的一个泛型版本:

public class TestGerner {    
    public static void main(String[] args) {
        
        List<Person> persons = new ArrayList<>();
        
        persons.add(new Person("AA",12));
        persons.add(new Person("BB",12));
        persons.add(new Person("CC",12));    

        Person person = persons.get(0);    //get的返回值就是Person类型      
    }
}

    我们说 List 是一个带一个类型参数的泛型接口，本例中，类型参数是 Integer。我们在创建这个 List 对象的时候也指定了一个类型参数。

/*
 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

/**
 * An ordered collection (also known as a <i>sequence</i>).  The user of this
 * interface has precise control over where in the list each element is
 * inserted.  The user can access elements by their integer index (position in
 * the list), and search for elements in the list.<p>
 *
 * Unlike sets, lists typically allow duplicate elements.  More formally,
 * lists typically allow pairs of elements <tt>e1</tt> and <tt>e2</tt>
 * such that <tt>e1.equals(e2)</tt>, and they typically allow multiple
 * null elements if they allow null elements at all.  It is not inconceivable
 * that someone might wish to implement a list that prohibits duplicates, by
 * throwing runtime exceptions when the user attempts to insert them, but we
 * expect this usage to be rare.<p>
 *
 * The <tt>List</tt> interface places additional stipulations, beyond those
 * specified in the <tt>Collection</tt> interface, on the contracts of the
 * <tt>iterator</tt>, <tt>add</tt>, <tt>remove</tt>, <tt>equals</tt>, and
 * <tt>hashCode</tt> methods.  Declarations for other inherited methods are
 * also included here for convenience.<p>
 *
 * The <tt>List</tt> interface provides four methods for positional (indexed)
 * access to list elements.  Lists (like Java arrays) are zero based.  Note
 * that these operations may execute in time proportional to the index value
 * for some implementations (the <tt>LinkedList</tt> class, for
 * example). Thus, iterating over the elements in a list is typically
 * preferable to indexing through it if the caller does not know the
 * implementation.<p>
 *
 * The <tt>List</tt> interface provides a special iterator, called a
 * <tt>ListIterator</tt>, that allows element insertion and replacement, and
 * bidirectional access in addition to the normal operations that the
 * <tt>Iterator</tt> interface provides.  A method is provided to obtain a
 * list iterator that starts at a specified position in the list.<p>
 *
 * The <tt>List</tt> interface provides two methods to search for a specified
 * object.  From a performance standpoint, these methods should be used with
 * caution.  In many implementations they will perform costly linear
 * searches.<p>
 *
 * The <tt>List</tt> interface provides two methods to efficiently insert and
 * remove multiple elements at an arbitrary point in the list.<p>
 *
 * Note: While it is permissible for lists to contain themselves as elements,
 * extreme caution is advised: the <tt>equals</tt> and <tt>hashCode</tt>
 * methods are no longer well defined on such a list.
 *
 * <p>Some list implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the list may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements in this list
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see Collection
 * @see Set
 * @see ArrayList
 * @see LinkedList
 * @see Vector
 * @see Arrays#asList(Object[])
 * @see Collections#nCopies(int, Object)
 * @see Collections#EMPTY_LIST
 * @see AbstractList
 * @see AbstractSequentialList
 * @since 1.2
 */

public interface List<E> extends Collection<E> {
    // Query Operations

    /**
     * Returns the number of elements in this list.  If this list contains
     * more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this list
     */
    int size();

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    boolean isEmpty();

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    boolean contains(Object o);

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    Iterator<E> iterator();

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must
     * allocate a new array even if this list is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in proper
     *         sequence
     * @see Arrays#asList(Object[])
     */
    Object[] toArray();

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to <tt>null</tt>.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of this list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of this list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);


    // Modification Operations

    /**
     * Appends the specified element to the end of this list (optional
     * operation).
     *
     * <p>Lists that support this operation may place limitations on what
     * elements may be added to this list.  In particular, some
     * lists will refuse to add null elements, and others will impose
     * restrictions on the type of elements that may be added.  List
     * classes should clearly specify in their documentation any restrictions
     * on what elements may be added.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this list
     */
    boolean add(E e);

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present (optional operation).  If this list does not contain
     * the element, it is unchanged.  More formally, removes the element with
     * the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list changed
     * as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this list
     */
    boolean remove(Object o);


    // Bulk Modification Operations

    /**
     * Returns <tt>true</tt> if this list contains all of the elements of the
     * specified collection.
     *
     * @param  c collection to be checked for containment in this list
     * @return <tt>true</tt> if this list contains all of the elements of the
     *         specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator (optional operation).  The behavior of this
     * operation is undefined if the specified collection is modified while
     * the operation is in progress.  (Note that this will occur if the
     * specified collection is this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    /**
     * Inserts all of the elements in the specified collection into this
     * list at the specified position (optional operation).  Shifts the
     * element currently at that position (if any) and any subsequent
     * elements to the right (increases their indices).  The new elements
     * will appear in this list in the order that they are returned by the
     * specified collection's iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.  (Note that this will occur if the specified
     * collection is this list, and it's nonempty.)
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt; size()</tt>)
     */
    boolean addAll(int index, Collection<? extends E> c);

    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection (optional operation).
     *
     * @param c collection containing elements to be removed from this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    /**
     * Retains only the elements in this list that are contained in the
     * specified collection (optional operation).  In other words, removes
     * from this list all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    /**
     * Removes all of the elements from this list (optional operation).
     * The list will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the <tt>clear</tt> operation
     *         is not supported by this list
     */
    void clear();


    // Comparison and hashing

    /**
     * Compares the specified object with this list for equality.  Returns
     * <tt>true</tt> if and only if the specified object is also a list, both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal</i>.  (Two elements <tt>e1</tt> and
     * <tt>e2</tt> are <i>equal</i> if <tt>(e1==null ? e2==null :
     * e1.equals(e2))</tt>.)  In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.  This
     * definition ensures that the equals method works properly across
     * different implementations of the <tt>List</tt> interface.
     *
     * @param o the object to be compared for equality with this list
     * @return <tt>true</tt> if the specified object is equal to this list
     */
    boolean equals(Object o);

    /**
     * Returns the hash code value for this list.  The hash code of a list
     * is defined to be the result of the following calculation:
     * <pre>
     *  int hashCode = 1;
     *  for (E e : list)
     *      hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
     * </pre>
     * This ensures that <tt>list1.equals(list2)</tt> implies that
     * <tt>list1.hashCode()==list2.hashCode()</tt> for any two lists,
     * <tt>list1</tt> and <tt>list2</tt>, as required by the general
     * contract of {@link Object#hashCode}.
     *
     * @return the hash code value for this list
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    int hashCode();


    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E get(int index);

    /**
     * Replaces the element at the specified position in this list with the
     * specified element (optional operation).
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the <tt>set</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E set(int index, E element);

    /**
     * Inserts the specified element at the specified position in this list
     * (optional operation).  Shifts the element currently at that position
     * (if any) and any subsequent elements to the right (adds one to their
     * indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt; size()</tt>)
     */
    void add(int index, E element);

    /**
     * Removes the element at the specified position in this list (optional
     * operation).  Shifts any subsequent elements to the left (subtracts one
     * from their indices).  Returns the element that was removed from the
     * list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E remove(int index);


    // Search Operations

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    int indexOf(Object o);

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    int lastIndexOf(Object o);


    // List Iterators

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * @return a list iterator over the elements in this list (in proper
     *         sequence)
     */
    ListIterator<E> listIterator();

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * The specified index indicates the first element that would be
     * returned by an initial call to {@link ListIterator#next next}.
     * An initial call to {@link ListIterator#previous previous} would
     * return the element with the specified index minus one.
     *
     * @param index index of the first element to be returned from the
     *        list iterator (by a call to {@link ListIterator#next next})
     * @return a list iterator over the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    ListIterator<E> listIterator(int index);

    // View

    /**
     * Returns a view of the portion of this list between the specified
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.  (If
     * <tt>fromIndex</tt> and <tt>toIndex</tt> are equal, the returned list is
     * empty.)  The returned list is backed by this list, so non-structural
     * changes in the returned list are reflected in this list, and vice-versa.
     * The returned list supports all of the optional list operations supported
     * by this list.<p>
     *
     * This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * <pre>
     *      list.subList(from, to).clear();
     * </pre>
     * Similar idioms may be constructed for <tt>indexOf</tt> and
     * <tt>lastIndexOf</tt>, and all of the algorithms in the
     * <tt>Collections</tt> class can be applied to a subList.<p>
     *
     * The semantics of the list returned by this method become undefined if
     * the backing list (i.e., this list) is <i>structurally modified</i> in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this list, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a view of the specified range within this list
     * @throws IndexOutOfBoundsException for an illegal endpoint index value
     *         (<tt>fromIndex &lt; 0 || toIndex &gt; size ||
     *         fromIndex &gt; toIndex</tt>)
     */
    List<E> subList(int fromIndex, int toIndex);
}

 

• 泛型类型参数或返回值

　　比如在Interface List<E> 下面会有两个方法（可以参见下面的泛型方法）

Object[]	toArray() Returns an array containing all of the elements in this list in proper sequence (from first to last element).
<T> T[]	toArray(T[] a) Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.

　　区别是什么呢？

public class TestGerner {    
    public static void main(String[] args) {
        
        List<Person> persons = new ArrayList<>();        
        persons.add(new Person("AA",12));
        persons.add(new Person("BB",12));
        persons.add(new Person("CC",12));        
        //这种写法是不对的，因为toArray()返回的是object类型，而object类型是不能转化为数组型的，除非一个个转化
        //Person [] personArray = (Person[]) persons.toArray();
        //System.out.println(personArray.length);
        
        //这种写法是对的，要传入一个泛型数组，只要是数组即可，所以new Person[0]是可以的
        //这样返回值就是对于泛型对应类型的一个数组
        Person [] personArray = persons.toArray(new Person[0]);
        System.out.println(personArray.length);
    }
}

 

• Map的遍历

      遍历Map的方式：

1. 得到键的集合，然后通过对键集合的遍历得到值
2. 直接得到值的集合
3. 泛型的方式

public class TestGerner {    
    public static void main(String[] args) {
        
        List<Person> persons = new ArrayList<>();        
        persons.add(new Person("AA",12));
        persons.add(new Person("BB",34));
        persons.add(new Person("CC",56));        
        
        Map<String,Person> personMap = new HashMap<String,Person>();
        personMap.put("AA",persons.get(0));
        personMap.put("BB",persons.get(1));
        personMap.put("CC",persons.get(2));
        
        for(Map.Entry<String,Person> entry: personMap.entrySet()){
            System.out.println( entry.getKey() + ":" + entry.getValue());
        }        
    }
}

结果是
AA:Person [name=AA, age=12]
BB:Person [name=BB, age=34]
CC:Person [name=CC, age=56]

 

• 定义简单的泛型类

　　类型参数在整个类的声明中可用，几乎是所有可以使用其他普通类型的地方都可以用

package com.atguigu.java;

public class DAO<T> {
    /**
     * 泛型类
     * 声明类的同时声明泛型类型
     * 1.方法的返回值可以是使用声明的泛型类型
     * 2.方法的参数也可以是声明类的泛型类型
     * 3.方法体内可以使用泛型类型
     */
    public T get(Integer id){
        return null;
    }
    
    public void save(T entity){        
    }
}

      类型参数就跟在方法或构造函数中普通的参数一样。就像一个方法有形式参数来描述它操作的参数的种类一样，一个泛型声明也有形式类型参数。当一个方法被调用，实参(actual arguments)替换形参，方法体被执行。当一个泛型声明被调用，实际类型参数(actual type arguments)取代形式类型参数。

     相当于声明的时候public class DAO<T>  T是一个形参，而使用的时候DAO<Person> dao = new DAO<>(); 相当于传入一个实参

 

• 泛型和子类继承

      如果 Foo 是 Bar 的一个子类型(子类或者子接口)，而 G 是某种泛型声明，那么 G<Foo>是 G<Bar>的子类型并不成立!!

public class TestGerner {    
    public static void main(String[] args) {
        //String 为 Object 类型的子类, 则 String[] 也是 Object[] 的子类
        Object [] objs = new String[]{"AA", "BB"};
                
        //String 为 Object 类型的子类, 则 List<String> 不是 List<Object> 的子类!
        //List<String> strList = new ArrayList<String>();
        //List<Object> objList = strList;
        //原因在于strList中存取的都是String，而objList中却可以存放非String类型，取出来的时候会有问题，与泛型基本原则相悖
        //泛型原则是某个集合对象只能放某个类型，不能放其他类型
        }
}

      为了处理这种情况，考虑一些更灵活的泛型类型很有用。

    通配符

     Collection<?>(发音为:"collection of unknown")，就是，一个集合，它的元素类型可以匹配任何类型。显然，它被称为通配符。

package com.atguigu.java;
//构造一个Student类，继承与Person

public class Student extends Person{
    private String school;

    public Student(String name,int age,String school) {
        super(name,age);
        this.school = school;
    }

    public String getSchool() {
        return school;
    }

    @Override
    public String toString() {
        return "Student [school=" + school + "]";
    }

    public void setSchool(String school) {
        this.school = school;
    }

}

public class TestGerner {    
    //定义一个打印信息方法
    public static void printPersonInfo(List<Person> persons){
        for(Person person:persons){
            System.out.println(person);
        }
    }
    public static void main(String[] args) {
         
        List<Person> persons = new ArrayList<>();        
        persons.add(new Person("AA",12));
        persons.add(new Person("BB",34));
        persons.add(new Person("CC",56));    
        //这里调用是没有问题的
        printPersonInfo(persons);
        
        List<Student> stus = new ArrayList<>();
        stus.add(new Student("TT",22,"TInghua"));
        //这里调用是不对的
        //因为Student是Person的子类，但是List<Student>却不是List<Person>的子类
        printPersonInfo(stus);
    }
}

　　　　把printPersonInfo改成通配符方法

//使用通配符方法
    public static void printPersonInfo(List<? extends Person> persons){
        for(Person person:persons){
            System.out.println(person);
        }
    }


/**
 * 结果是
 * Person [name=AA, age=12]
 * Person [name=BB, age=34]
 * Person [name=CC, age=56]
 * Student [school=TInghua]
 */

　　这样的话，printPersonInfo方法可以接受Person类以及Person子类

　　上面的例子是一个有上限通配符，如果没有上限，就是任意通配符

//任意通配符方法
    //取出来的时候，只能是Object类
    public static void printPersonInfo(List<?> persons){
        for(Object person:persons){
            System.out.println(person);
        }
    }

　　但是，将多种类型加入到同一个泛型集合中中不是类型安全的

　　add 方法有类型参数 E 作为集合的元素类型。我们传给 add 的任何参数都必须是一个未知类型的子类。因为我们不知道那是什么类型，所以我们无法传任何东西进去。唯一的例外是 null，它是所有类型的成员。

//通配符的安全性问题
    public static void printPersonInfo(List<? extends Person> persons){
        //加入null是可行的
        persons.add(null);
        //这句话编译出错，因为不能确定Person中到底是什么类型（可能是Person类型，也可能是其子类型）
        //所以往里面添加任何类型元素都是不行的
        persons.add(new Person("DD",78));
        
        //向外取数据是合法的
           //我们可以调用 get()方法并使用其返回值。返回值是一个未知的类型，但是我们知道，这里它总是一个Person   
        for(Person person:persons){
            System.out.println(person);
        }
    }

 

• 泛型方法

　　在非泛型类里面，可以单独的定义泛型方法

　　在泛型类里，泛型方法也可以使用类声明之外的泛型

package com.atguigu.java;

import java.util.ArrayList;
import java.util.Collection;

public class TestGerner {    
    /**
     * 泛型方法: 在方法声明时, 同时声明泛型. 在方法的返回值, 参数列表以及方法体中都可以使用泛型类型. 
     * public static <T> T get(Integer id){
     *      T result = null;        
     *      return result;
     * }
     * 把指定类型的数组中的元素放入到指定类型的集合中
     */
    public static <T> void fromArrayToCollection(T [] objs, Collection<T> coll){
        
    }
    
    public static void main(String[] args) {
    
        String[] objs2 = new String[]{"AA", "BB", "CC"};
        Collection<String> coll2 = new ArrayList<>();        
        fromArrayToCollection(objs2, coll2);
        
        Person[] objs3 = new Person[]{new Person("AA", 12)};
        Collection<Person> coll3 = new ArrayList<>();        
        fromArrayToCollection(objs3, coll3);
    }
}

 

实例化泛型对象的方法

http://blog.csdn.net/shigaofei1/article/details/6546416