[JAVA] What is the difference between interface and abstract? ?? ??

Roughly explain the difference

interface ・ ・ ・ Not a class (important)

abstract ・ ・ ・ Modifiers for classes, etc.

About interface

① Variable

Can declare and implement variables. However, "public static final" is implicitly added.

interface noTsukaikata {
	int Num = 6; //Since final is attached, it cannot be initialized. → It is treated as a constant.
}

② Method

Only abstract methods can be declared.

interface noTsukaikata {
	void method(); //Declaration of abstract method (method in interface is given abstract clause)
	// abstract void method();Is automatically converted to.
}

In fact, it can be implemented by using the default clause or static clause.

interface deJisou{
	//Implement methods in interface by using default clause
	default void method1(){
		//more codes here
	}

	//Implement methods in interface by using static clause
	static void method2(){
		//more codes here
	}

	//Compile error without default or static clause
	void method3() {
		//more codes here
	}
}

③ Inheritance method

interface can only inherit implements.

• All unimplemented methods in interface must be overridden.

interface Super{
	int Num;
	default void method1(){
		//more codes here.
	}
	static void method2(){
		//more codes here.
	}
	void method3();
}

class Sub implements Super{
	//The variable is final so you don't have to write it.

	//Since the default method has already been implemented, it is not necessary to implement it in the Sub class. Can be overridden as needed.

	//Since static methods are not inherited, they cannot be implemented in Sub class.

	//Implement method3
	@Override 
	protected void method3(){//Disclosure range(scope)Access modifiers may be changed as long as is not narrowed.
		//more codes here.
	}
}

The contents of the Sub class by inheriting the above are as follows.

class Sub implements Super{
	int Num = 6;
	default void method1(){
		//more codes here.
	}
	//There is no static method because it is not inherited.
	protected void method3(){
		//more codes here.
	}
}

About abstract

① Variable

Treated exactly the same as a regular class.

abstract class Super {
	int Num1;      //Can be declared
	int Num2 = 6;  //Can be initialized
	abstract int Num3 = 10;  //Compile error (variables cannot be abstracted)
}

② Method

Abstract methods can also be declared.

interface Super {
	abstract void method1(); //You can declare abstract methods.
	void method2() {}        //It can be implemented without the abstract clause.
}

③ Inheritance method

abstract can only inherit extends.

• All abstract methods must be overridden.

abstract class Super {
	int Num1;
	int Num2 = 6;
	abstract void method1();
	void method2() {}
}

class Sub extends Super{
	@Override 
	void method1() {}
}

The contents of the Sub class by inheriting the above are as follows.

class Sub extends Super{
	int Num1;
	int Num2 = 6;
	void method1() {}
	void method2() {}
}

What was the difference after all? ?? ??

Supplement

What is the difference between implements and extends? ?? ??

implements ・ ・ ・ Inherits interface. extends ・ ・ ・ Inherits the class.

Conclusion,

interface ・ ・ ・ Not a class

abstract ・ ・ ・ Modifiers for classes, etc.