Java | Фишки и трюки

public class Animal {
    public Animal eat() {
        System.out.println("animal eat");
        return null;
    }
    
    public Long calc() {
        return null;
    }
}

public class Dog extends Animal {
    public Dog eat() {
        return new Dog();
    }
    
/*attempting to use incompatible return type
    public Integer calc() {
        return null;
    }
*/
}

public class Quest {
    public static void main(String[] args) {
        System.out.println(!!true);
    }
}

// Для импорта конкретного статического члена
import static package.Clazz.member;

// Для импорта всех статических членов класса
import static package.Clazz.*;

public class MathUtils {
    public static final double PI = 3.141592653589793;
    public static int add(int a, int b) {
        return a + b;
    }
    public static int subtract(int a, int b) {
        return a - b;
    }
}

import static MathUtils.PI;
import static MathUtils.add;

public class Main {
    public static void main(String[] args) {
        System.out.println("Value of PI: " + PI);
        System.out.println("Addition result: " + add(5, 3));
    }
}

import static MathUtils.*;

public class Main {
    public static void main(String[] args) {
        System.out.println("Value of PI: " + PI);
        System.out.println("Addition result: " + add(5, 3));
        System.out.println("Subtraction result: " + subtract(5, 3));
    }
}

public class Quest {
    public static void main(String[] args) {
        Value val = new Value(1);
        System.out.print(val.i);
        System.out.print(change(val).i);
        System.out.print(val.i);
    }

    public static Value change(Value val) {
        val.i = 2;
        return val;
    }

    static class Value {
        int i;
        public Value(int i) {
            this.i = i;
        }
    }
}

public class Quest {

    public static void main(String[] args) {
        int i = 1;
        System.out.print(i);
        System.out.print(change(i));
        System.out.print(i);
    }

    public static int change(int i) {
        i = 2;
        return i;
    }

}

public class Alert {
    public Alert(String message) {
        System.out.println(message);
    }
}

public class Parent {
    public static Alert statVar1 = new Alert("Parent: static variable (statVar1)");
    public static Alert statVar2;
    static {
        statVar2 = new Alert("Parent: static initialization block (statVar2)");
    }

    public Alert var1 = new Alert("Parent: variable (var1)");
    public Alert var2;
    {
        var2 = new Alert("Parent: initialization block (var2)");
    }
    public Alert var3 = new Alert("Parent: variable (var3)");

    public static Alert statVar3 = new Alert("Parent: static variable (statVar3)");

    public Alert contructorVar;
    public Parent() {
        contructorVar = new Alert("Parent: constructor (contructorVar)");
    }
}

public class Child extends Parent {
    public static Alert statVar1 = new Alert("Child: static variable (statVar1)");
    public static Alert statVar2;
    static {
        statVar2 = new Alert("Child: static initialization block (statVar2)");
    }

    public Alert var1 = new Alert("Child: variable (var1)");
    public Alert var2;
    {
        var2 = new Alert("Child: initialization block (var2)");
    }
    public Alert var3 = new Alert("Child: variable (var3)");

    public static Alert statVar3 = new Alert("Child: static variable (statVar3)");

    public Alert contructorVar;
    public Child() {
        contructorVar = new Alert("Child: constructor (contructorVar)");
    }
}

public class Test {
    public static void main(String[] args) {
        new Child();
    }
}

Parent: static variable (statVar1)
Parent: static initialization block (statVar2)
Parent: static variable (statVar3)
Child: static variable (statVar1)
Child: static initialization block (statVar2)
Child: static variable (statVar3)
Parent: variable (var1)
Parent: initialization block (var2)
Parent: variable (var3)
Parent: constructor (contructorVar)
Child: variable (var1)
Child: initialization block (var2)
Child: variable (var3)
Child: constructor (contructorVar)

public class One {
    public static void go() {
        System.out.print("one ");
    }
}

public class Two extends One {
    public static void go() {
        System.out.print("two ");
    }
}

public class Three extends Two {
    public static void go() {
        System.out.print("three ");
    }
}

public class Quest {
    public static void main(String[] args) {
        One one = new One();
        one.go();

        One two = new Two();
        two.go();

        Two two2 = new Three();
        two2.go();
    }
}