1、Random
import java.util.Random;
int i = new Random().nextInt(4);
2、Math
int i = (int)(Math.random()*4); //Math.random = [0, 1)
1、Random
import java.util.Random;
int i = new Random().nextInt(4);
2、Math
int i = (int)(Math.random()*4); //Math.random = [0, 1)