Java 对象在 JVM 中的内存布局超详细解说

public class Test {
    public static void main(String[] args) {
        // 创建一个 ProgramLanguage 对象, 对象名是 java
        ProgramLanguage java = new ProgramLanguage();
        // 实例化一个 ProgramLanguage 对象, 对象名是 c
        ProgramLanguage c = new ProgramLanguage();
        // 把 ProgramLanguage 类实例化, 实例化后的对象的对象名是 python
        ProgramLanguage python = new ProgramLanguage();
    }
}

class ProgramLanguage {
    private Integer id;
    private String name;
}

public class DogDemo {
    public static void main(String[] args) {
        Dog doggy = new Dog();
        doggy.age = 6;
        doggy.weight = 3.14;
    }
}

public class Dog {
    public int price;
}

public class Person {
    public int age;
    public Dog dog;
}

public class Test {
    public static void main(String[] args) {
        Dog doggy = new Dog();
        doggy.price = 255;

        Person yeTing = new Person();
        yeTing.age = 20;
        yeTing.dog = doggy;
    }
}

public class Dog {
    public int price;

    public void run() {
        System.out.println(price   "_"   "run()");
    }

    public void eat() {
        System.out.println(price   "_"   "eat()");
    }
}

public class Test {
    public static void main(String[] args) {
        Dog dog1 = new Dog();
        dog1.price = 255;
        dog1.run();
        dog1.eat();

        Dog dog2 = new Dog();
        dog2.price = 552;
        dog2.run();
        dog2.eat();
    }
}

public class Dog {
    String name;
    int age;
    String color;
}

public class DogDemo {
    public static void main(String[] args) {
        Dog doggy = new Dog();
        doggy.name = "笑天";
        doggy.age = 6;
        doggy.color = "黑";
    }
}

/**
 * 访问修饰符有四种：public、protected、默认（不写）、private
 */
public class Dog {
    public String name;
    protected int age;
    String color;
    private double weight;
}

/**
 * 测试若不给对象的属性赋初始值, 它们的默认初始值
 */
public class FiledInitialValueTest {
    private int score;
    private short age;
    private byte month;
    private long salary;
    private float height;
    private double pi;
    private char firstName;
    private boolean isTrueLove;
    private Person person;

    public static void main(String[] args) {
        FiledInitialValueTest test = new FiledInitialValueTest();
        System.out.println("\n若不给对象的属性赋值, 初始值如下所示：");
        System.out.println("score【int】 = "   test.score);
        System.out.println("age【short】 = "   test.age);
        System.out.println("month【byte】 = "   test.month);
        System.out.println("salary【long】 = "   test.salary);
        System.out.println("height【float】 = "   test.height);
        System.out.println("pi【double】 = "   test.pi);
        // 字符类型的属性的初始值是空串（在控制台无法看到）
        System.out.println("firstName【char】 = "   test.firstName);
        // 字符类型的属性的初始值强制类型转换为 int 后是【0】
        System.out.println("firstName【(int)char】 = "   (int) test.firstName);
        System.out.println("isTrueLove【boolean】 = "   test.isTrueLove);
        System.out.println("person【person】 = "   test.person);
    }
}

public class TestCreateObject {
    public static void main(String[] args) {
        // (1) 先声明再创建
        Dog dog; // 声明 
        dog = new Dog(); // 通过 new 关键字创建对象

        // (2) 声明并创建对象 
        Dog doggy = new Dog();
    }
}

public class Person {
    private int age;
    private String name;

    public static void main(String[] args) {
        Person yangJiaLi = new Person();
        yangJiaLi.age = 17;
        yangJiaLi.name = "杨嘉立";

        // 下面的一行代码有2种说法：
        // 1. 把 yangJiaLi 赋值给 yjl
        // 2. yjl 指向 yangJiaLi
        Person yjl = yangJiaLi;
        System.out.println(yjl.age); // 17
    }
}