Java面向对象编程：构造函数链与接口详解

### Java面向对象编程：构造函数链与接口详解 #### 构造函数链：this()和super()的使用 在Java面向对象编程中，构造函数起着初始化对象的重要作用。构造函数不能被继承或重写，但可以在同一个类中进行重载。当为一个类定义多个构造函数时，由于构造函数的名称总是与类名相同，因此每个构造函数的参数列表必须不同。 ##### this()构造函数调用 `this()`构造函数调用用于在同一个类中实现构造函数的局部链式调用。以下是一个示例： ```java class Light { // 字段 private int noOfWatts; // 瓦数 private boolean indicator; // 开关状态 private String location; // 位置 // 构造函数 Light() { this(0, false); System.out.println("Returning from default constructor no. 1."); } Light(int watt, boolean ind) { this(watt, ind, "X"); System.out.println("Returning from non-default constructor no. 2."); } Light(int noOfWatts, boolean indicator, String location) { this.noOfWatts = noOfWatts; this.indicator = indicator; this.location = location; System.out.println("Returning from non-default constructor no. 3."); } } public class DemoThisCall { public static void main(String[] args) { System.out.println("Creating Light object no. 1."); Light light1 = new Light(); System.out.println("Creating Light object no. 2."); Light light2 = new Light(250, true); System.out.println("Creating Light object no. 3."); Light light3 = new Light(250, true, "attic"); } } ``` 在上述代码中，`Light`类有三个构造函数。默认构造函数`Light()`调用了`this(0, false)`，而`Light(int watt, boolean ind)`调用了`this(watt, ind, "X")`。当创建`Light`对象时，构造函数按照逆序执行，即先执行最深层的构造函数，最后执行最初调用的构造函数。 Java要求`this()`调用必须作为构造函数的第一条语句，这是为了处理子类对象创建时父类构造函数的调用机制。 以下是程序的输出： ```plaintext Creating Light object no. 1. Returning from non-default constructor no. 3. Returning from non-default constructor no. 2. Returning from default constructor no. 1. Creating Light object no. 2. Returning from non-default constructor no. 3. Returning from non-default constructor no. 2. Creating Light object no. 3. Returning from non-default constructor no. 3. ``` ##### super()构造函数调用 `super()`构造函数调用用于在子类构造函数中调用直接父类的构造函数。这使得子类在创建对象时能够影响其继承状态的初始化。以下是一个示例： ```java class Light { // 字段 private int noOfWatts; private boolean indicator; private String location; // 构造函数 Light() { this(0, false); System.out.println("Returning from default constructor no. 1 in class Light"); } Light(int watt, boolean ind) { this(watt, ind, "X"); System.out.println("Returning from non-default constructor no. 2 in class Light"); } Light(int noOfWatts, boolean indicator, String location) { super(); this.noOfWatts = noOfWatts; this.indicator = indicator; this.location = location; System.out.println("Returning from non-default constructor no. 3 in class Light"); } } class TubeLight extends Light { // 实例变量 private int tubeLength; private int colorNo; // 构造函数 TubeLight(int tubeLength, int colorNo) { this(tubeLength, colorNo, 100, true, "Unknown"); System.out.println("Returning from non-default constructor no. 1 in class TubeLight"); } TubeLight(int tubeLength, int colorNo, int noOfWatts, boolean indicator, String location) { super(noOfWatts, indicator, location); this.tubeLength = tubeLength; this.colorNo = colorNo; System.out.println("Returning from non-default constructor no. 2 in class TubeLight"); } } public class Chaining { public static void main(String[] args) { System.out.println("Creating a TubeLight object."); TubeLight tubeLightRef = new TubeLight(20, 5); } } ``` 在上述代码中，`TubeLight`类是`Light`类的子类。`TubeLight`的构造函数通过`super()`调用了`Light`类的相应构造函数。 以下是程序的输出： ```plaintext Creating a TubeLight object. Returning from non-default constructor no. 3 in class Light Returning from non-default constructor no. 2 in class TubeLight Returning from non-default constructor no. 1 in class ```