04.Java Basics - Classes

Basic concepts

#In the object-oriented concept, we know that all objects are represented by classes.

In Java, classes are defined by the keyword class.

Similarly, classes in Java have many forms, including ordinary classes, abstract classes, and inner classes.

Inner classes include: member inner classes, local inner classes, anonymous inner classes, and static inner classes.

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Normal category

There is nothing to say about this.

public class Demo { }

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Abstract class

1. Basic concept

All objects mentioned above are passed through classes To describe, but not all classes are used to describe objects.

If a class does not contain enough information to describe a specific object (we can understand it as an incomplete class), such a class is Abstract class.

Abstract classes are defined through the keyword absract. It can contain abstract methods and non-abstract methods.

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2. Example study

• Abstract class definition

public abstract class Parent {

    // 1.成员变量，与普通类无差别
    private  String word="aa";    // 2.非抽象方法，与普通类无差
    private void talk(){
        System.out.println("Parent is talking");
    }    // 3.抽象方法，访问权限只能是 public 和 protected 
    abstract void print();
}

• Abstract class inheritance

// 1.抽象类继承抽象类，抽象子类不用实现父类的方法public abstract class Son extends Parent { }
// 2.普通类继承抽象类，普通子类必须实现父类的所有抽象方法public class Grandson extends Son {
    @Override
    void print() {
        System.out.println("I am Grandson");
    }
}

• Abstract class call

// 错误，抽象类不允许被实例化//Parent parent = new Parent();

Through the above code, we can summarize the following points:

• Abstract classes and abstract methods cannot be modified by private and must use keywords abstract definition.

• If the subclass is not an abstract class, it must implement all abstract methods of the parent class.

• Abstract class is not allowed to be instantiated, compilation error.

• Abstract classes can also contain ordinary methods and member variables.

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Member inner class

1. Basic concepts

Located inside a class The class is called a member inner class.

Member inner classes have the following characteristics: all properties of their outer classes can be accessed without any special conditions.

2. Example study

• Member internal class definition:

public class Outter {
    private int a = 10;    static int b = 20;    int c = 30;    // 内部类
    class Inner {        void print(int d) {
            System.out.println(a + "-" +b+ "-" +c+ "-" + "-" +d;
        }
    }    // 取得内部类
    Inner getInner() {        return new Inner();
    }
}

• Member internal class call

Outter out = new Outter();// 创建内部类的两种方式：
Outter.Inner inner = out.new Inner();Outter.Inner inner2 = out.getInner();inner.print(20);

3. Principle analysis

By decompiling the class file, the command is as follows:

javap -v Outter

After executing the command, two class files are obtained: Oututter.Class and Oututter$Inner.Class.

Explanation: For virtual machines, internal classes are actually the same as regular classes. So Inner is still compiled into an independent class, rather than a certain field of the Outter class.

But because the member inner class looks like a member of the outer class, it can have the same access rights as the member.

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Local internal classes

1. Basic concepts

There are two types of local internal classes:

• Class within the method.

• Classes within the scope.

It can be regarded as an internal local variable of the method or scope, so its access rights are limited to the method or the scope.

Like local variables, it cannot be modified by the public, protected, private, and static keywords.

2. Example study

public class Man {
    public Object getWoman() {        // 注意：三个变量都相互不受影响
        int age = 30;        // 1.方法内的类
        class Woman {            int age = 20;
        }        // 2.作用域内的类，此时作用域为 if 
        if(true){
            class Son{                int age = 10;
            }
        }   
        return new Woman();
    }
}

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Anonymous inner class

1.Basic concepts

Anonymous inner classes are the only classes without a constructor. Because of this feature, the scope of use of anonymous inner classes is very limited, and most of them are used for interface callbacks.

It has the following characteristics:

• The anonymous inner class is automatically named Oututter$1.class by the system during compilation.

• Anonymous inner classes are generally used to inherit other classes or implement interfaces. There is no need to add additional methods, but only the implementation or rewriting of inherited methods.

• Anonymous inner classes cannot access local variables in outer class methods unless the variables are declared as final type

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2. Example study

public class Test {
    // a 属于全局变量
    int a =100;    public static void main(String[] args) {
        Test t = new Test();
        t.test(200);
    }    // b 属于局部变量
    public void test(final int b) {        // c 属于局部变量
        final int c = 300;        //  匿名内部类
        new Thread() {            int  d = 400;            public void run() {
                System.out.println(a+"-"+b+"-"+c+"-"+d);
            };
        }.start();
    }
}

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3. Principle analysis

You can get two class files through the decompile command: Outter.class and Oututter$1.class.

About the life cycle of local variables:

• When the method is called, local variables are created on the stack. When the method ends, it is popped off the stack and the local variables die.

About the declaration cycle of inner class objects:

• Create an anonymous inner class object, and the system allocates memory for the object until no variables are referenced. Points to the memory allocated to the object before it will be processed by GC.

So there is a situation:

• The member method has been called and the local variable has died, but the object of the anonymous inner class is still alive.

So the final keyword + copy method is used in Java to solve the problem:

• final 关键字：因为它的特性是一旦变量被赋值后，就不能被修改。

• 复制：在匿名内部类中直接复制了一个与局部变量值的数，让它变成自己的局部变量。

所以当局部变量的生命周期结束后，匿名内部类照样可以访问 final 类型的局部变量，因为它自己拷贝了一份，且与原局部变量的值始终一致。

下面针对上面的代码来分析：

• 当 test 方法执行完毕之后，变量 b、c 的生命周期就结束了。然而 Thread 对象的生命周期很可能还没有结束。因此要将 b、c 设置为 final 。

• a 之所以不采用 final 修饰，因为它是全局变量，生命周期是随着类的结束而结束。而类的生命周期肯定大于匿名内部类。

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静态内部类

1.基本概念

静态内部类也是定义在一个类里面的类，只不过在类的前面多了一个关键字static。

静态内部类和静态成员变量其实具有相同的特点：

• 它只有类有关，与对象无关。因此可以在没有外部类对象情况下，创建静态内部类。

• 不能访问外部类的非静态成员或方法

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2.实例探究

• 内部静态类定义：

public class Outter {
    int a = 5;    static int b = 500;    // 静态内部类
    static class Inner {        public Inner() {            // 只能访问外部类的静态成员
            System.out.println(b);
        }
    }
}

• 静态内部类调用：

// 静态内部类的调用不依赖外部类对象Outter.Inner inner= new Outter.Inner();

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接口内部类

1.基本概念

接口内部类，顾名思义就是在接口内定义的类。

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2.实例探究

• 接口内部类定义：

public interface People{
    class Man {        public void print() {
            System.out.println("man.print()...");
        }
    }
}

• 接口内部类调用：

People.Man  man = new People.Man();

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