Java笔记

初学Java，有些东西需要记录一下。

Part 1

Java的方法只能传值，不能传引用，但是可以用对象来解决这个问题。

例如swap函数：

方案1：构造一个新类MyInteger

public class SwapTwoNumbers {
    static class MyInteger {
        private int a;
        public MyInteger(int a) {
            this.a = a;
        }
        public int getValue() {
            return a;
        }
        public void changeValue(int a) {
            this.a = a;
        }
    }
    public static void swap(MyInteger a, MyInteger b) {
        int t = a.getValue();
        a.changeValue(b.getValue());
        b.changeValue(t);
    }
    public static void main(String[] args) {
        MyInteger a = new MyInteger(4);
        MyInteger b = new MyInteger(10);
        System.out.println("a is "+a.getValue()+" and b is "+b.getValue());
        swap(a, b);
        System.out.println("a is "+a.getValue()+" and b is "+b.getValue());
    }
}

方案2：利用main所在的类本身

public class t1 {
	int a, b;
	public t1(int a, int b) {
		this.a = a;
		this.b = b;
	}
	public void swap() {
		int t = a;
		a = b;
		b = t;
	}
	public static void main(String[] args) {
		t1 x = new t1(3,4);
		System.out.println("a is "+x.a+" and b is "+x.b);
		x.swap();
		System.out.println("a is "+x.a+" and b is "+x.b);
	}
}

方案3：利用数组

public static void swap(int[] data, int a, int b) {
    int t = data[a];
    data[a] = data[b];
    data[b] = t;
}

Part2

求gcd。

import java.util.Random;

public class t1 {
	public static int gcd(int a, int b) {
		//if(a <= 0 || b <= 0) return 0;
		if(a%b==0) return b;
		return gcd(b, a%b);
	}
	public static void main(String[] args) {
		Random r = new Random();
		int a = r.nextInt(), b = r.nextInt();
		a=Math.abs(a); b=Math.abs(b);
		System.out.println(a+" "+b);
		if(a<b) System.out.println(gcd(b,a));
		else System.out.println(gcd(a,b));
	}
}

求素数。