1、foreach介绍及其使用范围
foreach语句是java5的新特征之一，在遍历数组、集合、Iterable对象方面，为开发人员提供了极大的方便。foreach语句是for语句特殊情况下的增强版本，简化了编程，提高了代码的可读性和安全性（不用怕数组越界）。foreach语法支持对数组、Collection对象、Iterable对象的遍历。
2、foreach适用于对数组遍历
String[] array = {"A","B","C","D","E","F","G"};
for(String str:array){
System.out.print(str+" ");
}
3、foreach适用于对Collection对象遍历
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.TreeSet;
public class ForeachDemo {
public static void main(String[] args) {
String[] array = {"A","A","B","E","F","G","C","D"};
Collection<String> list = new ArrayList<String>(Arrays.asList(array));
for(String str:list){////按照被添加的顺序保存对象
System.out.print(str+" ");
}
System.out.println();
Collection<String> linkedList = new LinkedList<String>(Arrays.asList(array));
for(String str:linkedList){//按照被添加的顺序保存对象
System.out.print(str+" ");//A A B E F G C D
}
System.out.println();
Collection<String> hashSet = new HashSet<String>(Arrays.asList(array));
for(String str:hashSet){//无重复，无序,存储顺序并无实际意义
System.out.print(str+" ");//D E F G A B C
}
System.out.println();
Collection<String> treeSet = new TreeSet<String>(Arrays.asList(array));
for(String str:treeSet){//无重复，按照比较结果的升序保存对象
System.out.print(str+" ");//A B C D E F G
}
System.out.println();
Collection<String> linkedHashSet = new LinkedHashSet<String>(Arrays.asList(array));
for(String str:linkedHashSet){//无重复，按照被添加的顺序保存对象
System.out.print(str+" ");//A B E F G C D
}
}
}
代码优化，利用上转型抽象出公共方法：
public static void forEachCollection(Collection<String> colleciton){
for(String str:colleciton){////按照被添加的顺序保存对象
System.out.print(str+" ");
}
System.out.println();
}
测试用例：
public static void listCollection() {
String[] array = {"A","A","B","E","F","G","C","D"};
forEachCollection(new ArrayList<String>(Arrays.asList(array)));//A A B E F G C D
forEachCollection(new LinkedList<String>(Arrays.asList(array)));//按照被添加的顺序保存对象 //A A B E F G C D
forEachCollection(new HashSet<String>(Arrays.asList(array)));//无重复，无序,存储顺序并无实际意义//D E F G A B C
forEachCollection(new TreeSet<String>(Arrays.asList(array)));//无重复，按照比较结果的升序保存对象//A B C D E F G
forEachCollection(new LinkedHashSet<String>(Arrays.asList(array)));//无重复，按照被添加的顺序保存对象//A B E F G C D
}
3.1、利用迭代遍历Collection对象：
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.TreeSet;
public class ForeachDemo {
public static void printCollection(Collection<String> colleciton){
Iterator<String> it = colleciton.iterator();
while(it.hasNext()){
System.out.print(it.next()+" ");
}
System.out.println();
}
public static void main(String[] args) {
String[] array = {"A","A","B","E","F","G","C","D"};
printCollection(new ArrayList<String>(Arrays.asList(array)));//A A B E F G C D
printCollection(new LinkedList<String>(Arrays.asList(array)));//按照被添加的顺序保存对象 //A A B E F G C D
printCollection(new HashSet<String>(Arrays.asList(array)));//无重复，无序,存储顺序并无实际意义//D E F G A B C
printCollection(new TreeSet<String>(Arrays.asList(array)));//无重复，按照比较结果的升序保存对象//A B C D E F G
printCollection(new LinkedHashSet<String>(Arrays.asList(array)));//无重复，按照被添加的顺序保存对象//A B E F G C D
}
}
4、foreach适用于对Iterable对象遍历:
package com.andieguo.iterabledemo;
import java.util.Iterator;
public class IteratorSequence<T> {
private T[] array = null;
public IteratorSequence(T[] array){
this.array = array;
}
public Iterable<T> iterator(){//Iterable能够用于foreach
return new Iterable<T>(){
@Override
public Iterator<T> iterator() {
// TODO Auto-generated method stub
return new Iterator<T>() {
private int index = 0;
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
return index < array.length;
}
@Override
public T next() {
// TODO Auto-generated method stub
return array[index++];
}
@Override
public void remove() {
// TODO Auto-generated method stub
throw new UnsupportedOperationException();
}
};
}};
}
public static void main(String[] args) {
IteratorSequence<String> iteratorSequence = new IteratorSequence<String>(new String[]{"A","B","C"});
for(String str:iteratorSequence.iterator()){//iteratorSequence.iterator()返回的是一个Iterable<T>实例，支持foreach循环
System.out.print(str+" ");
}
}
}