Herein, preferably with sharp lesson you learn how the problem that occurs when a string comparison and the use of equal (=) operator to compare strings.
Introduction
Strings are Java in a special class. We Java regular use of the program String , therefore compare two strings are Java a common practice in. In this article, I tried to answer the most common questions about strings, for example: "How to compare Java string?"
In identity verification, sorting, citing matching process, comparing strings very helpful.
I have listed three of the more Java methods strings.
1. Use the equals () methods (Comparative content)
2. Use == operator (comparison reference)
3. Use the compareTo () method (lexicographic order string comparison)
1. Use Equals () method of comparing strings
In this way, I am using String class .equals () instance method. Initially .equals () method is the Object class methods, String class overrides it.
equals the value of equality () method of comparing two strings, regardless of whether they are logically equivalent.
String class the equals () method to another string as a parameter, and compares it with the specified string. If and only if the parameter string is not null time and includes the same character string specified, it returns to true .
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public boolean equals(Object anObject)
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It compare this string with the argument strings and return true if the argument is not null and contains the same character as the specified string.
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param -
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another string
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returns -
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true - if argument is not null and it contains same characters as the specified string
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false - if the argument is null or it does not contain same characters as the specified string
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ex. firstString.equals(secondString)
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// returns true if and only if the secondString is not null and contains the same characters as firstString.
I have asked the program using the following the equals () method to compare strings:
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/**
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* A Java program to compare two strings using equsls()
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* and equalsIgnoreCase() method of the String.
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*
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* @author Gaurav Kukade at coderolls.com
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*/
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public class CompareUsingEquals {
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public static void main(String[] args) {
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String firstString = "coderolls";
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String secondString = "javablog";
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String thirdString = "coderolls";
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String fourthString = "CodeRolls";
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System.out.println("Comparing strings using equals() and equalsIgnoreCase() method\n");
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// Using equals() method
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System.out.print("firstString.equals(secondString) : ");
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System.out.println(firstString.equals(secondString));
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System.out.print("firstString.equals(thirdString) : ");
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System.out.println(firstString.equals(thirdString));
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/*
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* Using equalsIgnoreCase() method to ignore
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* case consideration (i.e. Capital or small) of both the strings.
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*/
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System.out.print("firstString.equalsIgnoreCase(fourthString) : ");
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System.out.println(firstString.equalsIgnoreCase(fourthString));
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}
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}
Output:
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Comparing strings using equals() and equalsIgnoreCase() method
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firstString.equals(secondString) : false
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firstString.equals(thirdString) : true
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firstString.equalsIgnoreCase(fourthString) : true
2.使用==运算符比较字符串
在String中，==运算符用于比较给定字符串的引用，具体取决于它们是否引用相同的对象。
当使用==运算符比较两个字符串时，如果字符串变量指向同一个Java对象，它将返回true。否则，它将返回false。
我给了一个Java程序，可以在下面使用==运算符进行比较：
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/**
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* A Java program to compare strings using == operator.
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*
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* == operator ckecks whether both the strings referring
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* to the same String Object.
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*
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* @author Gaurav Kukade at coderolls.com
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*/
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public class CompareUsingEqualsToOperator {
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public static void main(String[] args) {
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String firstString = "coderolls";
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String secondString = "javablog";
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String thirdString = "coderolls";
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// creating new String object with the same value as firstString or thirdString
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String fourthString = new String("coderolls");
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System.out.println("Comparing strings using == operator \n");
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System.out.print("firstString == secondString : ");
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System.out.println(firstString == secondString);
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/*
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* Here firstString and thirdString is referring to the same String object
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* hence it will print 'true'.
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*/
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System.out.print("firstString == thirdString : ");
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System.out.println(firstString == thirdString);
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/*
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* Here firstString and fourthString have same value
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* but they are referring to the different String object.
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*
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* hence it will print 'false'
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*/
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System.out.print("firstString == fourthString : ");
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System.out.println(firstString == fourthString);
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}
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}
Output:
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Comparing strings using == operator
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firstString == secondString : false
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firstString == thirdString : true
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firstString == fourthString : false
使用==运算符进行字符串比较时出现问题
大多数初学者Java开发人员通过使用== operator比较两个字符串来犯此错误。
从逻辑上讲，他们必须检查两个字符串是否包含相同的字符序列。
在Java字符串中，==运算符用于检查两个字符串对象的引用，而equals（）方法用于检查两个字符串的值相等。
== –检查引用是否相等
equals（）–检查值是否相等
当我们为字符串变量分配字符串值时，JVM将检查字符串池中是否已经存在具有相等值的字符串。 如果它在字符串池中不存在，它将被添加到常量池中，并返回对该字符串对象的引用。
如果它存在于字符串池中，则返回对该字符串对象的内存地址的引用。
下图显示了相同的图片说明。
上面，我们有“ firstString”指向字符串池中的“ coderolls”字符串。
如果我们将相等的值分配给另一个字符串变量，则JVM将检查具有该值的字符串是否存在于字符串常量池中。
由于具有该值的字符串对象已在上一步中创建，因此另一个字符串变量开始引用先前创建的字符串对象实例。
下图显示了指向字符串池中“ coderolls”字符串的“ firstString”和“ secondString”的图形说明。
当我们使用new运算符创建字符串时，将创建一个新的字符串对象并将其存储在Java堆空间中。
在上方，我们可以看到“ firstString”和“ secondString”指向字符串池中的“ coderolls”字符串，而“ thirdString”指向Java堆空间中的“ coderolls”。
如果参数字符串在字典上大于指定的字符串，即参数字符串在指定的字符串之后，则返回负整数。（参数String>指定的String）
如果参数字符串在字典上小于指定的字符串，即参数字符串在指定的字符串之前，则返回正整数。（参数String <指定的String）
如果两个字符串在字典上都相等，则返回零。（参数String =指定的String）
如果要忽略两个字符串的大小写，请使用compareToIgnoreCase（）方法。
我提供了一个使用compareTo（）方法比较字符串的程序。它还包括一个用compareToIgnoreCase（）方法忽略大小写的大小写。
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/**
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* A Java program to compare strings using compareTo()
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* and compareToIgnoreCase() method.
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*
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* compareTo() compare strings lexicograpgically.
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*
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* @author Gaurav Kukade at coderolls.com
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*/
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public class CompareUsingCompareTo {
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public static void main(String[] args) {
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String firstString = "java";
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String secondString = "coderolls";
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String thirdString = "sql";
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String fourthString = "CodeRolls";
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System.out.println("Comparing strings using compareTo() and compareToIgnoreCase() method\n");
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// Using compareTo() method
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System.out.print("firstString.compareTo(secondString) : ");
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System.out.println(firstString.compareTo(secondString));
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System.out.print("firstString.compareTo(thirdString) : ");
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System.out.println(firstString.compareTo(thirdString));
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/*
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* Using compareToIgnoreCase() method to ignore
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* case consideration (i.e. Capital or small) of both the strings.
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*/
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System.out.print("secondString.compareToIgnoreCase(fourthString) : ");
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System.out.println(secondString.compareToIgnoreCase(fourthString));
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}
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}
Output:
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Comparing strings using compareTo() and compareToIgnoreCase() method
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firstString.compareTo(secondString) : 7
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firstString.compareTo(thirdString) : -9
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secondString.compareToIgnoreCase(fourthString) : 0
结论
我们可以使用以下给出的方法比较字符串：
1. 使用equals（）方法：用于检查字符串值是否相等的字符串中的equals（）方法是否包含相同的字符序列。
2. 使用==运算符：==运算符用于检查两个字符串的引用相等性，即它们是否指向同一字符串对象。
3. 使用compareTo（）方法：compareTo（）方法用于按字典顺序（即按字母顺序）检查字符串。查看有关如何按字典顺序比较字符串的详细文章。
大多数初学者Java开发人员在比较字符串时都会出错。他们想检查字符串的内容，但是他们使用==运算符检查它。
始终建议使用equals（）方法根据字符串的内容比较字符串。
如果你对上面给出的代码块有任何疑问，请在下面的注释部分写下来。另外，在注释部分中，让我知道你是否还有其他方法可以比较Java中的两个字符串。
感谢阅读！
另外近期整理了一套完整的java架构思维导图，分享给同样正在认真学习的每位朋友~
