2D List Iterator, Java

Language: Layout:

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
        List<Integer> list1 = new ArrayList<>(Arrays.asList(1, 2));
        List<Integer> list2 = new ArrayList<>(Arrays.asList(3));
        List<List<Integer>> vec2d = new ArrayList<>();
        vec2d.add(list1);
        vec2d.add(list2);
        //1,2
        //3
        MyIterator myIter = new MyIterator(vec2d);
        System.out.println(myIter.hasNext());
        System.out.println(myIter.next());
        System.out.println(myIter.hasNext());
        myIter.remove();
        System.out.println(myIter.next());
        for (int i = 0; i < vec2d.size(); i++) {
            if (vec2d.get(i) == null) continue;
            for (int j = 0; j < vec2d.get(i).size(); j++) {
                System.out.println("List" + i + ":" + vec2d.get(i).get(j));
            }
            
        }
        System.out.println(myIter.hasNext());
        System.out.println(myIter.next());
    }
}

class MyIterator implements Iterator<Integer> {
    private Iterator<List<Integer>> i;
    private Iterator<Integer> j;
    
    public MyIterator(List<List<Integer>> vec2d) {
        this.i = vec2d.iterator();
        this.j = null;
    }
    
    public boolean hasNext() {
        //注意这里是while不是if
        while ((j == null || !j.hasNext()) && i.hasNext()) {
            //如果有需要，比如List里面含有null元素，则需要特殊处理一下handle有null的情况
            //否则下面j = i.next().iterartor()就会null pointer
           j  = i.next().iterator();
        }
        return j != null && j.hasNext();
    }
    
    public Integer next() {
        if (!hasNext()) {
            return null;
        }
        return j.next();
    }
    
    public void remove() {
        //注意这里!j.hasNext()无所谓，只要判断j不是null就行
        while (j == null && i.hasNext()) {
            j  = i.next().iterator();
        }
        if (j != null) {
            j.remove();
        }
    }
}

[ + ] Show input
Compilation time: 1.74 sec, absolute running time: 0.27 sec, cpu time: 0.24 sec, memory peak: 18 Mb, absolute service time: 2,02 sec