20. java面向对象 - 构造器

一、定义

​ 构造器作用就是创建对象，或创建对象的同时为对象进行属性初始化，创建多个构造器以此构成重载。如果没有显示定义类的构造器的话，则系统默认提供一个空参构造器。new Person()。一旦我们显示定义了构造器，系统就不提供了默认构造器。

​ 构造器定义格式：修饰符 类名(形参列表)

1. 无参数

public class Person {
    String name;
    int age;
    public Person(){
        System.out.println("person()...");
    }
}


class PersonTest{
    public static void main(String[] args) {
        Person per = new Person();
        //person()...
    }
}

2. 有参数

public class Person {
    String name;
    int age;
    public Person(String n){
        name = n;
        System.out.println("person_name is " + name);
    }
}


class PersonTest{
    public static void main(String[] args) {
        Person per = new Person("tom");
        //person_name is tom
    }
}

二、实例

​ 编写两个类，TriangleTest和Triangle。其中Triangle类中声明私有的底边和高，同时声明公共方法去访问。此外提供必要的构造器，测试类求三角形面积。

Triangle

public class Triangle {
    private double base;
    private double height;

    //构造方法
    public Triangle() {

    }

    public Triangle(double b, double h) {
        base = b;
        height = h;
    }

    //私有属性公有化
    public void setBase(double b) {
        base = b;
    }

    public double getBase() {
        return base;
    }

    public void setHeight(double h) {
        height = h;
    }

    public double getHeight() {
        return height;
    }

    public double area(double b, double h) {
        return b * h / 2;
    }
}

TriangleTest

public class TriangleTest {
    public static void main(String[] args) {
        Triangle angle = new Triangle();
        double base = angle.getBase();
        double height = angle.getHeight();
        System.out.println("base is " + base + ", " + "height is " + height);

        Triangle angleTwo = new Triangle(10, 20);
        angleTwo.setHeight(100);
        // 500.0
        System.out.println("面积是" + angleTwo.area(angleTwo.getBase(), angleTwo.getHeight()));
    }
}