问题

如何在整数左填充0
举例 1 = “0001”

答案一，String.format

String.format("%05d", yournumber);

用0填充，总长度为5
https://docs.oracle.com/javase/8/docs/api/java/util/Formatter.html

答案二，ApacheCommonsLanguage

如果需要在Java 1.5前使用，可以利用 Apache Commons Language 方法

org.apache.commons.lang.StringUtils.leftPad(String str, int size, '0')

答案三，DecimalFormat

import java.text.DecimalFormat;
class TestingAndQualityAssuranceDepartment
{
    public static void main(String [] args)
    {
        int x=1;
        DecimalFormat df = new DecimalFormat("00");
        System.out.println(df.format(x));
    }
}

答案四，自己实现

如果效率很重要的话，相比于 String.format 函数的可以自己实现

/**
 * @param in The integer value
 * @param fill The number of digits to fill
 * @return The given value left padded with the given number of digits
 */
public static String lPadZero(int in, int fill){

    boolean negative = false;
    int value, len = 0;

    if(in >= 0){
        value = in;
    } else {
        negative = true;
        value = - in;
        in = - in;
        len ++;
    }

    if(value == 0){
        len = 1;
    } else{         
        for(; value != 0; len ++){
            value /= 10;
        }
    }

    StringBuilder sb = new StringBuilder();

    if(negative){
        sb.append('-');
    }

    for(int i = fill; i > len; i--){
        sb.append('0');
    }

    sb.append(in);

    return sb.toString();       
}

效率对比

public static void main(String[] args) {
    Random rdm;
    long start;

    // Using own function
    rdm = new Random(0);
    start = System.nanoTime();

    for(int i = 10000000; i != 0; i--){
        lPadZero(rdm.nextInt(20000) - 10000, 4);
    }
    System.out.println("Own function: " + ((System.nanoTime() - start) / 1000000) + "ms");

    // Using String.format
    rdm = new Random(0);        
    start = System.nanoTime();

    for(int i = 10000000; i != 0; i--){
        String.format("%04d", rdm.nextInt(20000) - 10000);
    }
    System.out.println("String.format: " + ((System.nanoTime() - start) / 1000000) + "ms");
}

结果
自己的实现：1697ms
String.format：38134ms

答案，Google Guava

Maven：

    <dependency>
         <artifactId>guava</artifactId>
         <groupId>com.google.guava</groupId>
         <version>14.0.1</version>
    </dependency>

样例：

Strings.padStart("7", 3, '0') returns "007"
Strings.padStart("2020", 3, '0') returns "2020"

注意：
Guava 是非常有用的库，它提供了很多有用的功能，包括了Collections, Caches, Functional idioms, Concurrency, Strings, Primitives, Ranges, IO, Hashing, EventBus等

stackoverflow原址：
http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left