Java中的每個類從根本上都是繼承Object，標準容器類自然也不。因此容器類都有toString()方法，並且覆寫了這個方法，使得它生成的String結果能夠表達容器自身，以及容器所包含的物件。例如ArrayList.toString()，它會遍歷ArrayList中包含的所有物件，呼叫每個元素上的toString()方法：
Generator介面：

package com.jxs.chapeter13;

/**
 * Created by jiangxs on 2018/4/8.
 */
public interface Generator<T> {

    T next();
}
 

Coffee.java

package com.jxs.chapeter13;

/**
 * Created by jiangxs on 2018/4/8.
 */
public class Coffee {

    private static long counter = 0;
    private final long id = counter++;

    @Override
    public String toString() {

        return getClass().getSimpleName() + " " + id;
    }
}

    class Latte extends Coffee {
    }

    class Mocha extends Coffee {
    }

    class Cappuccino extends Coffee {
    }

    class Americano extends Coffee {
    }

    class Breve extends Coffee {
    }
 

CoffeeGenerator.java

package com.jxs.chapeter13;

import java.util.Iterator;
import java.util.Random;

/**
 * Created by jiangxs on 2018/4/8.
 */
public class CoffeeGenerator implements Generator<Coffee>,Iterable<Coffee> {

    private Class[] types = {Latte.class, Mocha.class,
            Cappuccino.class, Americano.class, Breve.class};
    private
 
 static Random random = new Random(47);

    public CoffeeGenerator() {
    }

    private int size;

    public CoffeeGenerator(int size) {
        this.size = size;
    }

    @Override
    public Coffee next() {

        try {
            return (Coffee) types[random.nextInt(types.length)].newInstance();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    class CoffeeIterator implements Iterator<Coffee> {

        int count = size;

        @Override
        public boolean hasNext() {

            return count > 0;
        }

        @Override
        public Coffee next() {
            count--;
            return CoffeeGenerator.this.next();
        }

        @Override
        public void remove() {

            throw new UnsupportedOperationException();
        }
    }

    @Override
    public Iterator<Coffee> iterator() {
        return new CoffeeIterator();
    }

    public static void main(String[] args) {

        CoffeeGenerator gen = new CoffeeGenerator();
        for (int i = 0; i < 5; i++) {
            System.out.println(gen.next());
        }
        for (Coffee c : new CoffeeGenerator(5)) {
            System.out.println(c);
        }
    }
}

ArrayListDisplay.java

package com.jxs.chapeter13;

import java.util.ArrayList;

/**
 * Created by jiangxs on 2018/4/8.
 */
public class ArrayListDisplay {

    public static void main(String[] args) {

        ArrayList<Coffee> coffees = new ArrayList<>();
        for (Coffee coffee : new CoffeeGenerator(10)) {
            coffees.add(coffee);
        }
        System.out.println(coffees);
    }
}

輸出結果：

[Americano 0, Latte 1, Americano 2, Mocha 3, Mocha 4, Breve 5, Americano 6, Latte 7, Cappuccino 8, Cappuccino 9]

Process finished with exit code 0

　　如果你需要toString()方法打印出物件的記憶體地址，也許你會考慮使用this這個關鍵字：
InfiniteRecursion.java

package com.jxs.chapeter13;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by jiangxs on 2018/4/8.
 */
public class InfiniteRecursion {

    @Override
    public String toString() {
        return " InfiniteRecursion address: " + this + "\n";
    }

    public static void main(String[] args) {

        List<InfiniteRecursion> v = new ArrayList<>();
        for (int i=0;i<10;i++) {
            v.add(new InfiniteRecursion());
        }
        System.out.println(v);
    }
}

　　當你建立了InfiniteRecursion物件，並將其打印出來的時候，你會得到一串非常長的異常。如果你將該InfiniteRecursion物件存入一個ArrayList中，然後列印該ArrayList，你也會得到同樣的異常：

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:125)
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:448)
    at java.lang.StringBuilder.append(StringBuilder.java:136)
    at com.jxs.chapeter13.InfiniteRecursion.toString(InfiniteRecursion.java:13)
    at java.lang.String.valueOf(String.java:2994)
···

其實，當代碼執行時：

"InfiniteRecursion address: " + this

　　這裡發生了自動型別轉換，由InfiniteRecursion型別轉換成String型別。因為編譯器看到一個String物件後面跟著一個”+”，而後面的物件不是String，於是編譯器試著將this轉換成一個String。它怎麼轉換呢，正是通過呼叫this上的toString()方法，於是就發生了遞迴呼叫。
　　如果你真的想要打印出物件的記憶體地址，應該呼叫Object.toString()方法，這才是負責此任務的方法。所以，你不該使用this，而是應該呼叫super.toString()方法。
如下InfiniteRecursionModify.java所示：

package com.jxs.chapeter13;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by jiangxs on 2018/4/8.
 */
public class InfiniteRecursionModify {

    @Override
    public String toString() {
        return " InfiniteRecursion address: " + super.toString() + "\n";
    }

    public static void main(String[] args) {

        List<InfiniteRecursionModify> v = new ArrayList<>();
        for (int i=0;i<10;i++) {
            v.add(new InfiniteRecursionModify());
        }
        System.out.println(v);
    }
}

輸出結果：

[ InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
,  InfiniteRecursion address: [email protected]
]

Process finished with exit code 0

參考：《Java程式設計思想》