java Iterator 的用法

java.util package has public interface Iterator and contains three methods:

1. boolean hasNext(): It returns true if Iterator has more element to iterate.
2. Object next(): It returns the next element in the collection until the hasNext()method return true. This method throws ‘NoSuchElementException’ if there is no next element.
3. void remove(): It removes the current element in the collection. This method throws ‘IllegalStateException’ if this function is called before next( ) is invoked.

// Java code to illustrate the use of iterator

import java.io.*;

import java.util.*;

 

class Test {

    public static void main(String[] args)

    {

        ArrayList<String> list = new ArrayList<String>();

 

        list.add("A");

        list.add("B");

        list.add("C");

        list.add("D");

        list.add("E");

 

        // Iterator to traverse the list

        Iterator iterator = list.iterator();

 

        System.out.println("List elements : ");

 

        while (iterator.hasNext())

            System.out.print(iterator.next() + " ");

 

        System.out.println();

    }

}

ListIterator

‘ListIterator’ in Java is an Iterator which allows users to traverse Collection in both direction. It contains the following methods:

1. void add(Object object): It inserts object immediately before the element that is returned by the next( ) function.
2. boolean hasNext( ): It returns true if the list has a next element.
3. boolean hasPrevious( ): It returns true if the list has a previous element.
4. Object next( ): It returns the next element of the list. It throws ‘NoSuchElementException’ if there is no next element in the list.
5. Object previous( ): It returns the previous element of the list. It throws ‘NoSuchElementException’ if there is no previous element.
6. void remove( ): It removes the current element from the list. It throws ‘IllegalStateException’ if this function is called before next( ) or previous( ) is invoked.

// Java code to illustrate the use of ListIterator

import java.io.*;

import java.util.*;

 

class Test {

    public static void main(String[] args)

    {

        ArrayList<String> list = new ArrayList<String>();

 

        list.add("A");

        list.add("B");

        list.add("C");

        list.add("D");

        list.add("E");

 

        // ListIterator to traverse the list

        ListIterator iterator = list.listIterator();

 

        // Traversing the list in forward direction

        System.out.println("Displaying list elements in forward direction : ");

 

        while (iterator.hasNext())

            System.out.print(iterator.next() + " ");

 

        System.out.println();

 

        // Traversing the list in backward direction

        System.out.println("Displaying list elements in backward direction : ");

 

        while (iterator.hasPrevious())

            System.out.print(iterator.previous() + " ");

 

        System.out.println();

    }

}

 

 

详见https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

// Java code to illustrate the use of ListIterator

import java.io.*;

import java.util.*;

 

class Test {

    public static void main(String[] args)

    {

        ArrayList<String> list = new ArrayList<String>();

 

        list.add("A");

        list.add("B");

        list.add("C");

        list.add("D");

        list.add("E");

 

        // ListIterator to traverse the list

        ListIterator iterator = list.listIterator();

 

        // Traversing the list in forward direction

        System.out.println("Displaying list elements in forward direction : ");

 

        while (iterator.hasNext())

            System.out.print(iterator.next() + " ");

 

        System.out.println();

 

        // Traversing the list in backward direction

        System.out.println("Displaying list elements in backward direction : ");

 

        while (iterator.hasPrevious())

            System.out.print(iterator.previous() + " ");

 

        System.out.println();

    }

}