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);
}
}
}
