JAVA基础--容器 Set, List, Map
Colections接口, Iterator接口, Set接口, List接口, Comparable接口, Map接口
Collections类
容器:装各种对象. 所有容器都在java.util里. Set, List, Map
1136:
1一个图:
Collection接口分为Set和List两个接口
Set接口: 没有顺序并且不可以重复 equals判断是否可重复
List接口: 有顺序并且可以重复
Map接口定义了存储 key-value映射对的方法
Collection接口
Set接口 List接口 Map接口
HashSet LinkedList ArrayList HashMap
1个类:Collections
3个知识点:for, generic, auto-boxing/unboxing
6个接口: Collection, Set,List, Map, Iterator, Comparable.
Collection接口所定义的方法:
size(), isEmpty(),clear(),contains(), add(), remove(),iterator(),containsAll(), addAll(), removeAll(),retainAll(), toArray()
容器类对象在调用remove,contains等方法时需要比较对象是否相等, 就会涉及到equals和hashCode方法.对于自定义的类需要重写这两个方法.
hashCode会用于对象用于索引的时候, 比如Map的key值.
Collection方法举例:
import java.util.*; class Name{ private String firstName, lastName; public Name(String firstName, String lastName){ this.firstName=firstName; this.lastName=lastName; } public String getFirstName(){ return firstName; } public String getLastName(){ return lastName; } public String toString(){ return firstName+" "+lastName; } public boolean equals(Object obj){ if(obj instanceof Name){ Name name =(Name) obj; return (firstName.equals(name.firstName))&&(lastName.equals(name.lastName)); } return super.equals(obj); } public int hashCode(){ return firstName.hashCode(); } } public class Test{ public static void main(String[] args){ Collection c = new ArrayList(); //父类引用调用子类对象 c.add("hello"); c.add(new Name("f1","l1")); c.add(new Integer(100)); //不用基本类型的原因, int类型数据是存放在栈里的 c.remove("hello"); c.remove(new Integer(100)); System.out.println(c.size()); System.out.println(c.remove(new Name("f1","l1"))); //重写equals方法,所以可以删掉上面add的那个 System.out.println(c); } }
Iterator接口: 容器里的元素挨个取出来. 最常用!!!!!
public class Test { public static void main(String[] args){ Collection c = new HashSet(); c.add(new Name("f1","l1")); c.add(new Name("f2","l2")); c.add(new Name("f3","l3")); Iterator i = c.iterator(); while(i.hasNext()){ Name n = (Name)i.next(); System.out.println(n.getFirstName()+" "+n.getLastName()); } } }
remove()方法是迭代过程中删除元素唯一安全的方法 :
public class Test { public static void main(String[] args){ Collection c = new HashSet(); c.add(new Name("fff1","lll1")); c.add(new Name("f2","l2")); c.add(new Name("ffff3","llll3")); for(Iterator i = c.iterator(); i.hasNext();){ Name n = (Name)i.next(); if(n.getFirstName().length()<3){ i.remove(); //不能用c.remove(name); , 会产生例外 } } System.out.println(c); } }
for循环用于遍历容器集合,但是除了简单遍历并读出其中内容外, 不建议使用增强for:
import java.util.*; public class EnhanceFor { public static void main(String[] args){ int[] arr={1,2,3,4,5}; for(int i:arr){ System.out.println(i); } Collection c =new ArrayList(); c.add(new String("aaa")); c.add(new String("bbb")); c.add(new String("ccc")); for(Object o : c){ System.out.println(o); } } }
缺点:
1.数组: 无法访问下标值.
2. 集合: 与使用Iterator相比, 不能方便删除集合中的内容.
Set接口的容器类:Hashset:
public class Test{ public static void main(String[] args){ Set s=new HashSet(); //父类引用调用子类对象 s.add("hello"); s.add("world"); s.add(new Name("f1","l1")); s.add(new Integer(100)); //不用基本类型的原因, int类型数据是存放在栈里的 s.add(new Name("f1","l1")); //重复元素不会加入 s.add("hello"); //重复元素不会加入
System.out.println(s); } }
public class Test{ public static void main(String[] args){ Set s1=new HashSet(); //父类引用调用子类对象 Set s2=new HashSet(); //父类引用调用子类对象 s1.add("a"); s1.add("b"); s1.add("c"); s2.add("d"); s2.add("a"); s2.add("b"); Set sn=new HashSet(s1); sn.retainAll(s2); //求交集 Set su=new HashSet(s1); su.addAll(s2); //加起来, 重复去掉 System.out.println(sn); System.out.println(su); } }
List接口的容器类: ArrayList, LinkedList
List方法举例:
import java.util.*; public class Test { public static void main(String[] args){ List l1 = new LinkedList(); for(int i = 0;i<=5;i++){ l1.add("a"+i); } System.out.println(l1); l1.add(3,"a100"); System.out.println(l1); l1.set(6,"a200"); System.out.println(l1); System.out.println((String)l1.get(2)+" "); System.out.println(l1.indexOf("a3")); l1.remove(1); System.out.println(l1); } }
结果:
[a0, a1, a2, a3, a4, a5] [a0, a1, a2, a100, a3, a4, a5] [a0, a1, a2, a100, a3, a4, a200] a2 4 [a0, a2, a100, a3, a4, a200]
一个类:Collections: 提供一些静态方法实现基于List容器的算法
还有其他方法:
void sort(List): 排序
void shuffle(List):随机排序
void reverse(List): 逆序排序
void fill(List,Object), 用一个特定对象重写整个List容器
void copy(List dest,List src);
int binarySearch(List,Object); 折半查找
import java.util.*; public class Test { public static void main(String[] args){ List l1 = new LinkedList(); List l2 = new LinkedList(); for(int i = 0;i<=9;i++){ l1.add("a"+i); } System.out.println(l1); Collections.shuffle(l1); System.out.println(l1); Collections.reverse(l1); System.out.println(l1); Collections.sort(l1); System.out.println(l1); System.out.println(Collections.binarySearch(l1,"a5")); } }
结果:
[a0, a1, a2, a3, a4, a5, a6, a7, a8, a9] [a1, a5, a3, a2, a7, a0, a9, a8, a6, a4] [a4, a6, a8, a9, a0, a7, a2, a3, a5, a1] [a0, a1, a2, a3, a4, a5, a6, a7, a8, a9] 5
对类进行排序, 两个对象可以比较大小: Comparable接口: java.lang.Comparable
此接口只有一个方法:
public int compareTo(Object obj);
import java.util.*; class Name implements Comparable{ private String firstName, lastName; public Name(String firstName, String lastName){ this.firstName=firstName; this.lastName=lastName; } public String getfirstName(){ return firstName; } public String getlastName(){ return lastName; } public String toString(){ return firstName+" "+lastName; } public boolean equals(Object obj){ if(obj instanceof Name){ Name name =(Name) obj; return (firstName.equals(name.firstName))&&(lastName.equals(name.lastName)); } return super.equals(obj); } public int hashCode(){ return firstName.hashCode(); } public int compareTo(Object o){ Name n = (Name) o; int lastCmp=lastName.compareTo(n.lastName); return(lastCmp!=0?lastCmp:firstName.compareTo(n.firstName)); } } public class Test{ public static void main(String[] args){ List l1=new LinkedList(); l1.add(new Name("Karl","M")); l1.add(new Name("Steven","Lee")); l1.add(new Name("John","O")); l1.add(new Name("Tom","M")); System.out.println(l1); Collections.sort(l1); //调用上面的compareTo方法 System.out.println(l1); } }
如何衡量标准:读和改的效率:
Array:读快改慢
Linked:改快读慢
Hash:两者之间
Map接口:实现类: HashMap, TreeMap
Map中的键值通过键来标示, 键值不能重复, 必须重写equals和hashCode方法
常用方法:
Object put(Object key, Object value); //如果放入了已经存在的一个key, 那么新的value会替换掉原来的value, 所以会返回原来的value Object get(Object key); //通过key找到value Object remove(Object key); boolean containsKey(Object key); boolean containsValue(Object value); int size(); boolean isEmpty(); void putAll(Map t); void clear();
Map方法举例:
import java.util.*; public class EnhanceFor { public static void main(String[] args){ Map m1=new HashMap(); Map m2=new TreeMap(); m1.put("one",new Integer(1)); m1.put("two",new Integer(2)); m1.put("three",new Integer(3)); m2.put("A",new Integer(1)); m2.put("B",new Integer(2)); System.out.println(m1.size()); System.out.println(m1.containsKey("one")); System.out.println(m2.containsValue(new Integer(1))); if(m1.containsKey("two")){ int i=((Integer)m1.get("two")).intValue(); System.out.println(i); } Map m3=new HashMap(m1); m3.putAll(m2); System.out.println(m3); } }
自动Boxing和Unboxing后上述程序可以改成这样:
import java.util.*; public class TestMap { public static void main(String[] args){ Map m1=new HashMap(); Map m2=new TreeMap(); m1.put("one",1); //自动boxing m1.put("two",2); m1.put("three",3); m2.put("A",1); m2.put("B",2); System.out.println(m1.size()); System.out.println(m1.containsKey("one")); System.out.println(m2.containsValue(1)); if(m1.containsKey("two")){ int i=(Integer)m1.get("two"); System.out.println(i); } Map m3=new HashMap(m1); m3.putAll(m2); System.out.println(m3); } }
结果:
3 true true 2 {two=2, A=1, B=2, one=1, three=3}
泛型: 定义集合的时候同时定义集合中对象的类型, 可以在定义Collection的时候指定, 也可以在循环的时候用Iterator指定, 这样可以增强程序的可读性和稳定性
public class Test{ public static void main(String[] args){ List<String> c=new ArrayList<String>(); c.add("aaa"); c.add("bbb"); c.add("ccc"); for(int i=0;i<c.size();i++){ String s=c.get(i); System.out.println(s); } Collection<String> c2=new HashSet<String>(); c2.add("aaa"); c2.add("bbb"); c2.add("ccc"); for(Iterator<String> it=c2.iterator();it.hasNext();){ String s=it.next(); System.out.println(s); } } }
用泛型改写Map的例子:
import java.util.*; public class TestMap { public static void main(String[] args){ Map<String,Integer> m1=new HashMap<String,Integer>(); m1.put("one",1); m1.put("two",2); m1.put("three",3); System.out.println(m1.size()); System.out.println(m1.containsKey("one")); if(m1.containsKey("two")){ int i=m1.get("two"); System.out.println(i); } } }