Java里如何将一个字符串重复n次 （新）

【说明】

以下代码提供了两种方案，一种是基于java的Collections.nCopies实现的，普适性好，效率稍低；一种是基于字符串数组拼接实现的，效率稍高。

各位请自行选择。

【代码】

import java.util.Collections;

public class Test {
    public static void main(String[] args) {
        // 示例用法
        System.out.println("(repeatString1)星号重复4次:"+repeatString1("*",4));
        System.out.println("(repeatString2)星号重复4次:"+repeatString2("*",4));
        System.out.println("(repeatString1)abcde_重复5次:"+repeatString1("abcde_",5));
        System.out.println("(repeatString2)abcde_重复5次:"+repeatString2("abcde_",5));

        // 效率比较
        long startMs=System.currentTimeMillis();
        for(int i=0;i<100000;i++) {
            repeatString1("abcde",4);
        }
        long endMs=System.currentTimeMillis();
        System.out.println("repeatString1运行十万次耗时:"+ms2DHMS(startMs,endMs));

        startMs=System.currentTimeMillis();
        for(int i=0;i<100000;i++) {
            repeatString2("abcde",4);
        }
        endMs=System.currentTimeMillis();
        System.out.println("repeatString2运行十万次耗时:"+ms2DHMS(startMs,endMs));
    }

    /**
     * 重复种子N次，耗时稍长
     * @param seed
     * @param n
     * @return
     */
    public static String repeatString1(String seed, int n) {
        return String.join("", Collections.nCopies(n, seed));
    }

    /**
     * 重复种子N次，耗时稍短
     * @param seed
     * @param n
     * @return
     */
    public static String repeatString2(String seed, int n){
        final int seedLen=seed.length();

        final char[] srcArr=seed.toCharArray();
        char[] dstArr=new char[n*seedLen];

        for(int i=0;i<n;i++){
            for(int j=0;j<seedLen;j++){
                dstArr[i*seedLen+j]=srcArr[j];
            }
        }

        return String.valueOf(dstArr);
    }

    private static String ms2DHMS(long startMs, long endMs) {
        String retval = null;
        long secondCount = (endMs - startMs) / 1000;
        String ms = (endMs - startMs) % 1000 + "ms";

        long days = secondCount / (60 * 60 * 24);
        long hours = (secondCount % (60 * 60 * 24)) / (60 * 60);
        long minutes = (secondCount % (60 * 60)) / 60;
        long seconds = secondCount % 60;

        if (days > 0) {
            retval = days + "d" + hours + "h" + minutes + "m" + seconds + "s";
        } else if (hours > 0) {
            retval = hours + "h" + minutes + "m" + seconds + "s";
        } else if (minutes > 0) {
            retval = minutes + "m" + seconds + "s";
        } else if(seconds > 0) {
            retval = seconds + "s";
        }else {
            return ms;
        }

        return retval + ms;
    }
}

【运行效果】

(repeatString1)星号重复4次:****
(repeatString2)星号重复4次:****
(repeatString1)abcde_重复5次:abcde_abcde_abcde_abcde_abcde_
(repeatString2)abcde_重复5次:abcde_abcde_abcde_abcde_abcde_
repeatString1运行十万次耗时:78ms
repeatString2运行十万次耗时:15ms

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