Java编程思想学习笔记

Java编程思想学习笔记#

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无意识的递归

Java中的每个类从根本上都是继承自Object，标准容器类自然也不例外。因此容器类都有toString()方法，并且覆写了该方法，使得它生成的String结果能够表达容器自身，以及容器所包含的对象。例如ArrayList.toString()，它会遍历ArrayList中包含的所有对象，调用每个元素上的toString()方法：

//: ArrayListDisplay.java
public class ArrayListDisplay {
  public static void main(String[] args) {
    ArrayList<Coffee> coffees = new ArrayList<Coffee>();
    for(Coffee c : new CoffeeGenerator(10))
      coffees.add(c);
    System.out.println(coffees);
  }
}

输出结果：

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

相关程序：

class Coffee {
  private static long counter = 0;
  private final long id = counter++;
  public String toString() {
    return getClass().getSimpleName() + " " + id;
  }
}

class Latte extends Coffee {}
class Cappuccino extends Coffee {}
class Mocha extends Coffee {}
class Americano extends Coffee {}
class Breve extends Coffee {}

interface Generator<T> { T next(); }

class CoffeeGenerator implements Generator<Coffee>, Iterable<Coffee> {
  private Class[] types = {
    Latte.class, Mocha.class, Cappuccino.class, Americano.class, Breve.class, };
  private static Random rand = new Random(47);
  public CoffeeGenerator() {}
  // For iteration:
  private int size = 0;
  public CoffeeGenerator(int sz) { size = sz; } 
  public Coffee next() {
    try {
      return (Coffee)
        types[rand.nextInt(types.length)].newInstance();
      // Report programmer errors at run time:
    } catch(Exception e) {
      throw new RuntimeException(e);
    }
  }
  class CoffeeIterator implements Iterator<Coffee> {
    int count = size;
    public boolean hasNext() { return count > 0; }
    public Coffee next() {
      count--;
      return CoffeeGenerator.this.next();
    }
    public void remove() { // Not implemented
      throw new UnsupportedOperationException();
    }
  }; 
  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);
  }
}

如果希望toString()方法打印出对象的内存地址，也许会考虑使用this关键字：

//: InfiniteRecursion.java
import java.util.*;

public class InfiniteRecursion {
  public String toString() {
    return " InfiniteRecursion address: " + this + "\n";
  }
  public static void main(String[] args) {
    List<InfiniteRecursion> v = new ArrayList<InfiniteRecursion>();
    for(int i = 0; i < 10; i++)
      v.add(new InfiniteRecursion());
    System.out.println(v);
  }
}

当创建了InfiniteRecursion对象，并将其打印出来的时候，将会得到一串非常长的异常。如果你将该InfiniteRecursion对象存入一个ArrayList中，然后打印该ArrayList，你也会得到同样的异常。其实，当如下代码运行时：

" InfiniteRecursion address: " + this + "\n"

这里发生了自动类型转换，由InfiniteRecursion类型转换成String类型。因为编译器看到一个String对象后面跟着一个“+”，而再后面的对象不是String，于是编译器试着将this转换成一个String。它怎么转换呢，正是通过调用this上的toString()方法，于是就发生了递归调用。

如果真的想要打印出对象的内存地址，应该调用Object.toString()方法，这才是负责此任务的方法。所以，你不该使用this，而是应该调用super.toString()方法。
练习2：（1）修复InfiniteRecursion.java

public class E02_RepairInfinite {
  public String toString() {
    return " E02_RepairInfinite address: " + super.toString() + "\n";
  }
  public static void main(String[] args) {
    List<E02_RepairInfinite> v = new ArrayList<E02_RepairInfinite>();
    for(int i = 0; i < 10; i++)
      v.add(new E02_RepairInfinite());
    System.out.println(v);
  }
}

输出结果：

[ E02_RepairInfinite address: strings.E02_RepairInfinite@1fb8ee3
,  E02_RepairInfinite address: strings.E02_RepairInfinite@61de33
,  E02_RepairInfinite address: strings.E02_RepairInfinite@14318bb
,  E02_RepairInfinite address: strings.E02_RepairInfinite@ca0b6
,  E02_RepairInfinite address: strings.E02_RepairInfinite@10b30a7
,  E02_RepairInfinite address: strings.E02_RepairInfinite@1a758cb
,  E02_RepairInfinite address: strings.E02_RepairInfinite@1b67f74
,  E02_RepairInfinite address: strings.E02_RepairInfinite@69b332
,  E02_RepairInfinite address: strings.E02_RepairInfinite@173a10f
,  E02_RepairInfinite address: strings.E02_RepairInfinite@530daa
]