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);
}
}
}