Java編程思想學習筆記

Java編程思想學習筆記

目錄

字符串

字符串

無心識的遞歸

Java中的每一個類從根本上都是繼承自Object，標準容器類天然也不例外。所以容器類都有toString()方法，而且覆寫了該方法，使得它生成的String結果可以表達容器自身，以及容器所包含的對象。例如ArrayList.toString()，它會遍歷ArrayList中包含的全部對象，調用每一個元素上的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);
  }
}

輸出結果：

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

相關程序：

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);
  }
}

若是但願toString()方法打印出對象的內存地址，也許會考慮使用this關鍵字：

//: 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);
  }
}

當建立了InfiniteRecursion對象，並將其打印出來的時候，將會獲得一串很是長的異常。若是你將該InfiniteRecursion對象存入一個ArrayList中，而後打印該ArrayList，你也會獲得一樣的異常。其實，當以下代碼運行時：

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

這裏發生了自動類型轉換，由InfiniteRecursion類型轉換成String類型。由於編譯器看到一個String對象後面跟着一個「+」，而再後面的對象不是String，因而編譯器試着將this轉換成一個String。它怎麼轉換呢，正是經過調用this上的toString()方法，因而就發生了遞歸調用。

若是真的想要打印出對象的內存地址，應該調用Object.toString()方法，這纔是負責此任務的方法。因此，你不應使用this，而是應該調用super.toString()方法。
練習2：（1）修復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);
  }
}

輸出結果：

[ 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
]