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Merge branch 'master' of https://github.com/YuxiangQue/stackoverflow-java-top-qa into YuxiangQue-master
# Conflicts: # contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md
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#如何用0向左补齐一个整数?
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## 如何在整数左填充0
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## 问题
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### 问题
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如何在整数左填充0
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举例 1 = "0001"
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在Java中如何把一个整数转化为字符串并且用0向左补齐呢?
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我基本上是希望把一个不大于9999的整数用0补齐 (比如说1="0001")。
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## 解答
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### 答案一,String.format
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String.format("%05d", yournumber);
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可以将一个五位数用0补齐。
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用0填充,总长度为5
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https://docs.oracle.com/javase/8/docs/api/java/util/Formatter.html
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### 答案二,ApacheCommonsLanguage
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如果需要在Java 1.5前使用,可以利用 Apache Commons Language 方法
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org.apache.commons.lang.StringUtils.leftPad(String str, int size, '0')
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### 答案三,DecimalFormat
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import java.text.DecimalFormat;
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class TestingAndQualityAssuranceDepartment
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{
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public static void main(String [] args)
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{
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int x=1;
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DecimalFormat df = new DecimalFormat("00");
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System.out.println(df.format(x));
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}
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}
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### 答案四,自己实现
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如果效率很重要的话,相比于 String.format 函数的可以自己实现
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/**
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* @param in The integer value
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* @param fill The number of digits to fill
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* @return The given value left padded with the given number of digits
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*/
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public static String lPadZero(int in, int fill){
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boolean negative = false;
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int value, len = 0;
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if(in >= 0){
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value = in;
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} else {
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negative = true;
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value = - in;
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in = - in;
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len ++;
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}
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if(value == 0){
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len = 1;
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} else{
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for(; value != 0; len ++){
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value /= 10;
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}
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}
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StringBuilder sb = new StringBuilder();
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if(negative){
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sb.append('-');
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}
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for(int i = fill; i > len; i--){
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sb.append('0');
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}
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sb.append(in);
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return sb.toString();
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}
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效率对比
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public static void main(String[] args) {
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Random rdm;
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long start;
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// Using own function
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rdm = new Random(0);
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start = System.nanoTime();
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for(int i = 10000000; i != 0; i--){
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lPadZero(rdm.nextInt(20000) - 10000, 4);
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}
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System.out.println("Own function: " + ((System.nanoTime() - start) / 1000000) + "ms");
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// Using String.format
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rdm = new Random(0);
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start = System.nanoTime();
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for(int i = 10000000; i != 0; i--){
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String.format("%04d", rdm.nextInt(20000) - 10000);
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}
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System.out.println("String.format: " + ((System.nanoTime() - start) / 1000000) + "ms");
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}
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结果
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自己的实现:1697ms
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String.format:38134ms
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### 答案,Google Guava
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Maven:
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<dependency>
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<artifactId>guava</artifactId>
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<groupId>com.google.guava</groupId>
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<version>14.0.1</version>
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</dependency>
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样例:
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Strings.padStart("7", 3, '0') returns "007"
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Strings.padStart("2020", 3, '0') returns "2020"
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注意:
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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
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https://docs.oracle.com/javase/8/docs/api/java/util/Formatter.html
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Stackoverflow链接:http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left
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stackoverflow原址:
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http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left
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## 在调用 instanceof 前需要进行null检查吗
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### 问题:
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null instanceof SomeClass 会返回 null 还是抛出 NullPointerException 异常
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### 答案一
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在调用 instanceof 前不要进行null检查
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null instanceof SomeClass 会返回 null
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在 Java Language Specification 中 http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2
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```
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在运行时,如果该instanceof运算符的关系表达式(RelationExpression)不为 null,且这个引用可以被成功转型( §15.16),不抛出ClassCastException,则结果为true;
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否则结果为false。
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```
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### 答案二
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public class IsInstanceOfTest {
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public static void main(final String[] args) {
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String s;
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s = "";
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System.out.println((s instanceof String));
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System.out.println(String.class.isInstance(s));
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s = null;
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System.out.println((s instanceof String));
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System.out.println(String.class.isInstance(s));
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}
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}
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打印出
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true
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true
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false
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false
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### 原文链接
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http://stackoverflow.com/questions/2950319/is-null-check-needed-before-calling-instanceof
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