package demo;public class Singleton {private static final Singleton INSTANCE = new Singleton();private int a = 0;private Singleton(){}public static synchronized Singleton getInstance(){return INSTANCE;}public void methodA(){a++;System.out.println("a = "+a);}public static void main(String[] args){Singleton sg1 = Singleton.getInstance();Singleton sg2 = Singleton.getInstance();sg1.methodA();sg2.methodA();System.out.println(sg1 == sg2);}
}

package demo;
import java.util.Scanner;
public class Demo {public static void main(String[] args){Scanner sc = new Scanner(System.in);int n = sc.nextInt();Demo d = new Demo();System.out.println(n+" != "+d.factor(n));System.out.println("max = " + Long.MAX_VALUE); //输出long型的最大数值
    }public static long factor(int n){if(n == 0) return 1;elsereturn factor(n-1)*n;}
}

package demo;
public class Demo {public static long fib(int n){if(n == 1 || n == 2)return 1;elsereturn  fib(n-1)+fib(n-2);}public static void main(String[] args){for(int i = 1; i <= 20; i++){System.out.println("fib("+i+") = "+fib(i)); //嵌套输出
        }}
}

package demo;public class MemberTest {public static void main(String[] args){Member m1 = new Member();Member m2 = new Member();m1.setInstanceVar(100);             m2.setInstanceVar(200);Member.setClassVar(300);System.out.println("m1.instanceVar = "+m1.getInstanceVar());System.out.println("m1.classVar = "+Member.getClassVar());System.out.println("m2.instanceVar = "+m2.getInstanceVar());System.out.println("m2.classVar = "+Member.getClassVar());}
}class Member {int instanceVar;static int classVar;void setInstanceVar(int i){instanceVar = i;classVar = i; //实例方法可以访问静态变量
    }int getInstanceVar(){return instanceVar;}static void setClassVar(int i){classVar = i;//instanceVar = i;该方法错误，静态方法不能访问实例变量
    }static int getClassVar(){return classVar;}
}

package demo;public class Demo {int a, b;Demo(int x, int y){a = x;b = y;}Demo(int x){a = x;b = 0;}Demo(){this(1, 0); //this关键字的另一个用途是在构造方法中调用该类的另一个构造方法
    }public void init(int x){a = x;int b = 5;System.out.println("a = "+a+", this.b = "+this.b+", b = "+b);}public void display(){System.out.println("a = "+a+", b = "+b);}public static void main(String[] args){Demo demo = new Demo();demo.display();demo.init(100);                              Demo demo2 = new Demo(6, 8);demo2.display();}
}

package demo;public class PassByValue {public void change(int y){y = y*2;System.out.println("y = "+y);}public void change(Circle cc){cc.setRadius(100);}public static void main(String[] args){PassByValue pv = new PassByValue();int x = 100;pv.change(x);System.out.println("x = "+x);Circle cc;cc = new Circle(10);System.out.println("c的半径"+cc.getRadius());pv.change(cc);System.out.println("c的半径"+cc.getRadius());}
}package demo;
public class Circle{double radius;Circle(int x){radius = x;}public void setRadius(double r){radius = r;}public double getRadius(){return radius;}public double perimeter(){return 2*Math.PI*radius;}public double area(){return Math.PI*radius*radius;}
}

package demo;
/** 静态变量：不管有多少个对象都只有一个静态变量，所有对象公用这个一个静态变量，* 每个对象对静态变量的改变都会影响到其他的对象* * 访问：可以通过实例的名字来访问静态变量，但这种方法可能产生混乱的代码，*         最好的方法是通过类名访问静态变量* * 其中这个例子中没构造一个对象就调用依次构造函数，构造函数中的静态变量x就加一，*         所以通过最终得到的x可以知道创建了多少个对象* */
public class Demo {public static void main(String[] args){Counter c1 = new Counter();System.out.println(c1.x);Counter c2 = new Counter();System.out.println(c1.x);c1.x = 100;System.out.println(c2.x);   Counter.x = 10000;System.out.println(c2.x);System.out.println(Counter.x);}
}package demo;public class Counter {int y;static int x;public Counter(){x++;}
}

package demo;public class Someclass {static int i = 48;int j = 5;public static void display(){i = i + 100;System.out.println("i = "+i);/*j = j + 5;System.out.println("j = "+j);这两句话会产生编译错误，因为静态的方法成员不能访问非静态的变量成员，非静态的变量成员只能通过实例来访问*/}
}

package demo;
public class Demo {public static void main(String[] args){Circle cc;cc = new Circle();cc.setRadius(10);System.out.println("半径 = "+cc.radius);System.out.println("周长= "+cc.perimeter());System.out.println("面积 = "+cc.area());}
}
class Circle {double radius;public void setRadius(double r){radius = r;}public double getRadius(){return radius;}public double perimeter(){return Math.PI * 2 * radius;}    public double area(){return Math.PI * radius * radius;}
}

package demo;public class OverloadDemo {public void show(){System.out.println("No parameters.");}public void show(int a, int b){System.out.println("a = "+a+" b = "+b);}public void show(double d){System.out.println("d = "+d);}public void show(int a){System.out.println("a = "+a);}public static void main(String[] args){OverloadDemo od = new OverloadDemo();od.show();od.show(1, 2);od.show(1212);od.show(1.2345);}
}