int[] a = new int[]{1, 1, 1, 1};
        outer: while(true){
            System.out.println(Arrays.toString(a));
            int j = 3;
            while((a[j]+=1)==4){
                a[j]=1;
                j--;
                if (j<0) break outer;
            }
        }

------解决方案--------------------
乱写的，没这么简单吧？
for (int n1 = 1; n1 <= 3; n1++)
			for (int n2 = 1; n2 <= 3; n2++)
				for (int n3 = 1; n3 <= 3; n3++)
					for (int n4 = 1; n4 <= 3; n4++) {
						System.out.print(n1);
						System.out.print(n2);
						System.out.print(n3);
						System.out.print(n4);
						System.out.println();
					}

------解决方案--------------------
1.先得到无重复key,1，2，3的key就是int[]keys=new int[]{1,2,3}
2.位数是四位,全排列的数一共有keys.length的位数次方,也就是3*3*3*3个
3.用一个数number表示这个排列顺序,number%keys.length表示个位的值为keys[number%keys.length]
十位为keys[number/keys.length%keys.length]
千位为keys[number/keys.length/keys.length%keys.length]
number的范围为0 到 (3*3*3*3-1)
最终一个for就可以搞定