初始化和清除

class Rock {
  Rock() { // This is the constructor
    System.out.println("Creating Rock");
  }
}

public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock();
  }
} ///:~


class Rock {
  Rock(int i) {
    System.out.println(
      "Creating Rock number " + i);
  }
}

public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock(i);
  }
}

[b][方法过载/b]

import java.util.*;

class Tree {
  int height;
  Tree() {
    prt("Planting a seedling");
    height = 0;
  }
  Tree(int i) {
    prt("Creating new Tree that is "
        + i + " feet tall");
    height = i;
  }
  void info() {
    prt("Tree is " + height
        + " feet tall");
  }
  void info(String s) {
    prt(s + ": Tree is "
        + height + " feet tall");
  }
  static void prt(String s) {
    System.out.println(s);
  }
}

public class Overloading {
  public static void main(String[] args) {
    for(int i = 0; i < 5; i++) {
      Tree t = new Tree(i);
      t.info();
      t.info("overloaded method");
    }
    // Overloaded constructor:
    new Tree();
  }
} ///:~
区分过载方法

//: OverloadingOrder.java
// Overloading based on the order of
// the arguments.

public class OverloadingOrder {
  static void print(String s, int i) {
    System.out.println(
      "String: " + s +
      ", int: " + i);
  }
  static void print(int i, String s) {
    System.out.println(
      "int: " + i +
      ", String: " + s);
  }
  public static void main(String[] args) {
    print("String first", 11);
    print(99, "Int first");
  }
} ///:~

this关键字只能用于那些特殊的类——需明确使用当前对象的句柄。例如，假若您希望将句柄返回给当前对象，那么它经常在return语句中使用。


//: Leaf.java
// Simple use of the "this" keyword

public class Leaf {
  private int i = 0;
  Leaf increment() {
    i++;
    return this;
  }
  void print() {
    System.out.println("i = " + i);
  }
  public static void main(String[] args) {
    Leaf x = new Leaf();
    x.increment().increment().increment().print();
  }
} ///:~

尽管可用this调用一个构建器，但不可调用两个。由于自变量s的名字以及成员数据s的名字是相同的，所以会出现混淆。为解决这个问题，可用this.s来引用成员数据

public class Flower {
  private int petalCount = 0;
  private String s = new String("null");
  Flower(int petals) {
    petalCount = petals;
    System.out.println(
      "Constructor w/ int arg only, petalCount= "
      + petalCount);
  }
  Flower(String ss) {
    System.out.println(
      "Constructor w/ String arg only, s=" + ss);
    s = ss;
  }
  Flower(String s, int petals) {
    this(petals);
//!    this(s); // Can't call two!
    this.s = s; // Another use of "this"
    System.out.println("String & int args");
  }
  Flower() {
    this("hi", 47);
    System.out.println(
      "default constructor (no args)");
  }
  void print() {
//!    this(11); // Not inside non-constructor!
    System.out.println(
      "petalCount = " + petalCount + " s = "+ s);
  }
  public static void main(String[] args) {
    Flower x = new Flower();
    x.print();
  }
} ///:~

成员初始化

class Measurement {
  boolean t;
  char c;
  byte b;
  short s;
  int i;
  long l;
  float f;
  double d;
  void print() {
    System.out.println(
      "Data type      Inital value\n" +
      "boolean        " + t + "\n" +
      "char           " + c + "\n" +
      "byte           " + b + "\n" +
      "short          " + s + "\n" +
      "int            " + i + "\n" +
      "long           " + l + "\n" +
      "float          " + f + "\n" +
      "double         " + d);
  }
}

public class InitialValues {
  public static void main(String[] args) {
    Measurement d = new Measurement();
    d.print();
    /* In this case you could also say:
    new Measurement().print();
    */
  }
} ///:~

输入结果如下：


Data type      Inital value
boolean        false
char
byte           0
short          0
int            0
long           0
float          0.0
double         0.0

初始化顺序

class Tag {
  Tag(int marker) {
    System.out.println("Tag(" + marker + ")");
  }
}

class Card {
  Tag t1 = new Tag(1); // Before constructor
  Card() {
    // Indicate we're in the constructor:
    System.out.println("Card()");
    t3 = new Tag(33); // Re-initialize t3
  }
  Tag t2 = new Tag(2); // After constructor
  void f() {
    System.out.println("f()");
  }
  Tag t3 = new Tag(3); // At end
}

public class OrderOfInitialization {
  public static void main(String[] args) {
    Card t = new Card();
    t.f(); // Shows that construction is done
  }
} ///:~

Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()