迭代器基础

一、迭代器的基本用法

1、terator的三个方法

next()获得序列的下一个元素

hasNext()判断序列是否还有下一个元素

remove()移除next最近返回的元素，必须在next()之后调用

实例：

 

import typeinfo.pets.*;
import java.util.*;
public class SimpleIterator {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        List<Pet> l = Pets.arrayList(7);
        Iterator<Pet> it = l.iterator();
        System.out.println("[1]: " + l);
        
        System.out.print("[2]: ");
        for(Pet p : l) {
            System.out.print(l.indexOf(p) + ":" + p + " ");
        }
        System.out.println();
        
        System.out.print("[3]: ");
        while(it.hasNext()) {
            System.out.print(it.next() + " ");
        }
        System.out.println();
        
        it = l.iterator();
        for(int i = 0; i < 4; i++) {
            it.next();
            it.remove();
        }
        System.out.println("[4]: " + l);
    }

}

 输出：

2、ListIterator

ListIterator只对List有效，可以前后遍历

方法：

• next()返回下一个元素，next指针后移，previous指针后移
• hasNext()列表中是否有未返回的元素
• nextIndex()返回next指针下标
• previous()返回前一个元素，previous指针前移，next指针前移
• hasPrevious()逆向遍历，是否还有没返回的元素
• previousIndex()返回previous指针下标
• remove()删除previous()或者next()最近返回的元素
• add()插入一个元素在next指针之前，previous指针之后

 

previous指针初始化时，就在next指针前面（即previous=-1  next = 0）

实例：

 

import java.util.*;
import typeinfo.pets.*;

public class ListIteration {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        List<Pet> l = Pets.arrayList(6);
        ListIterator<Pet> it = l.listIterator();
        System.out.println("[1]: " + l);
        
        //正向遍历列表
        System.out.print("[2]: ");
        while(it.hasNext()) {
            Pet p = it.next();
            System.out.print("(" + p + ", " + it.previousIndex() + ", " 
            + it.nextIndex() + ")  ");
        }
        System.out.println();
        
        //逆向遍历列表
        System.out.print("[3]: ");
        while(it.hasPrevious()) {
            Pet p = it.previous();
            System.out.print("(" + p + ", " + it.previousIndex() + ", " +
            it.nextIndex() + ") " );
        }
        System.out.println();
        
        //将2位置元素设置成r
        Rat r = new Rat();
        for(int i = 0; i < 3; i++) { it.next(); }
        it.set(r);
        System.out.println("[4]: " + l);
        
        //删除后三个元素
        it = l.listIterator();
        while(it.hasNext()) {
            it.next();
        }
        for(int i = 0; i < 3; i++) {
            it.previous();
            it.remove();
        }
        System.out.println("[5]: " + l);
        
        //在最后添加r
        it.add(r);
        System.out.println("[6]: " + l);
    }

}

 

输出：

 

 

 

3、任何实现Iterable的类都可以使用foreach语法

 

import java.util.*;
public class MyCollection implements Iterable<String>{

    private String[] words;
    private int size ;
    MyCollection(String ...args) {
        words = args;
        for(String s : words) {
            if(s == null) break;
            size++;
        }
    }
    
    public Iterator<String> iterator() {
        return new Iterator<String>() {
            private int next = 0;
            public boolean hasNext() {
                return next < size;
            }
            public String next() {
                return words[next++];
            }
            public void remove() {
                
            }
        };
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        MyCollection m = new MyCollection("MyCollection","is","good");
        for(String s : m) {
            System.out.print(s + " ");
        }
    }

}

 

输出：

 MyCollection is good