Random list random generation of non repeating numbers

 There is a need , You need to randomly generate six digits , from 100001 ---999999

 But without repeating , And it doesn't affect performance . If every time you are born, you can judge whether it is repeated , When enough times are generated , Will affect performance .

I want to open a thread , Handle it in advance . Generate random numbers in advance . Just use it directly

package cn.silence.random;

import cn.hutool.core.util.RandomUtil;

import java.util.ArrayList;
import java.util.List;

public class CodeRandom {

    private final List<Integer> list = new ArrayList<>(1000000);

    private int index = 0;

    public void start() {
        while (list.size() < 1000000) {
            int code = RandomUtil.randomInt(100000, 999999);
            if (!list.contains(code)) {
                list.add(code);
            }
        }
    }

    public Integer getCode() {
        if (index >= list.size()) {
            return RandomUtil.randomInt(100000, 999999);
        }
        return list.get(index++);
    }
}

     CodeRandom codeRandom = new CodeRandom();
            // Start thread 
            new Thread(codeRandom::start).start();

 

Take only , Don't delete , So thread safety is not involved . If you delete list Words , The number generated later may cause repetition