前言：==和equals這個兩個東西，經常放在一塊做比較，下面我也給出一個簡單的例子，對他倆進行一個簡單的比較，先看例子，然后在看結論。（實驗環境：win7+jdk7）
1：==和equals簡單比較的實例代碼如下
public class BooleanTestMain { public static void main(String [] args){ Boolean _Boolean1 = true; Boolean _Boolean2 = true; Boolean _Boolean3 = new Boolean(true); Boolean _Boolean4 = new Boolean(true); //輸入對象的hashCode和identityHashCode，當對應的類沒有重寫hashCode()方法時，hashCode和identityHashCode是一致的 System.out.println("_Boolean1.hashCode() is : "+_Boolean1.hashCode()+" System.identityHashCode(_Boolean1) is : "+ System.identityHashCode(_Boolean1)); System.out.println("_Boolean2.hashCode() is : "+_Boolean2.hashCode()+" System.identityHashCode(_Boolean2) is : "+ System.identityHashCode(_Boolean2)); System.out.println("_Boolean3.hashCode() is : "+_Boolean3.hashCode()+" System.identityHashCode(_Boolean3) is : "+ System.identityHashCode(_Boolean3)); System.out.println("_Boolean4.hashCode() is : "+_Boolean4.hashCode()+" System.identityHashCode(_Boolean4) is : "+ System.identityHashCode(_Boolean4)); //==比較的是對象引用地址值 System.out.println("_Boolean1==_Boolean2 is : "+(_Boolean1==_Boolean2)); System.out.println("_Boolean3==_Boolean4 is : "+(_Boolean3==_Boolean4)); System.out.println("_Boolean1==_Boolean3 is : "+(_Boolean1==_Boolean3)); //equals比較的是對象的值，如果對象沒有重寫equals的話，其作用和==一樣，因為Object類的equals也是通過==來比較的 System.out.println("_Boolean1.equals(_Boolean2) is : "+(_Boolean1.equals(_Boolean2))); System.out.println("_Boolean3.equals(_Boolean4) is : "+(_Boolean3.equals(_Boolean4))); System.out.println("_Boolean1.equals(_Boolean3) is : "+(_Boolean1.equals(_Boolean3))); } }
上例運行的結果如下：
"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.BooleanTestMain _Boolean1.hashCode() is : 1231 System.identityHashCode(_Boolean1) is : 1969344019 _Boolean2.hashCode() is : 1231 System.identityHashCode(_Boolean2) is : 1969344019 _Boolean3.hashCode() is : 1231 System.identityHashCode(_Boolean3) is : 1746959769 _Boolean4.hashCode() is : 1231 System.identityHashCode(_Boolean4) is : 756415799 _Boolean1==_Boolean2 is : true _Boolean3==_Boolean4 is : false _Boolean1==_Boolean3 is : false _Boolean1.equals(_Boolean2) is : true _Boolean3.equals(_Boolean4) is : true _Boolean1.equals(_Boolean3) is : true Process finished with exit code 0
2：Boolean類的部分源碼如下所示，重寫了Object的hashCode()和equals()方法
/** * Returns a hash code for this {@code Boolean} object. * * @return the integer {@code 1231} if this object represents * {@code true}; returns the integer {@code 1237} if this * object represents {@code false}. */ public int hashCode() { return value ? 1231 : 1237; } /** * Returns {@code true} if and only if the argument is not * {@code null} and is a {@code Boolean} object that * represents the same {@code boolean} value as this object. * * @param obj the object to compare with. * @return {@code true} if the Boolean objects represent the * same value; {@code false} otherwise. */ public boolean equals(Object obj) { if (obj instanceof Boolean) { return value == ((Boolean)obj).booleanValue(); } return false; }
3：Object類的equals()方法的源碼如下所示，也是通過==來比較的
/** * Indicates whether some other object is "equal to" this one. * <p> * The {@code equals} method implements an equivalence relation * on non-null object references: * <ul> * <li>It is <i>reflexive</i>: for any non-null reference value * {@code x}, {@code x.equals(x)} should return * {@code true}. * <li>It is <i>symmetric</i>: for any non-null reference values * {@code x} and {@code y}, {@code x.equals(y)} * should return {@code true} if and only if * {@code y.equals(x)} returns {@code true}. * <li>It is <i>transitive</i>: for any non-null reference values * {@code x}, {@code y}, and {@code z}, if * {@code x.equals(y)} returns {@code true} and * {@code y.equals(z)} returns {@code true}, then * {@code x.equals(z)} should return {@code true}. * <li>It is <i>consistent</i>: for any non-null reference values * {@code x} and {@code y}, multiple invocations of * {@code x.equals(y)} consistently return {@code true} * or consistently return {@code false}, provided no * information used in {@code equals} comparisons on the * objects is modified. * <li>For any non-null reference value {@code x}, * {@code x.equals(null)} should return {@code false}. * </ul> * <p> * The {@code equals} method for class {@code Object} implements * the most discriminating possible equivalence relation on objects; * that is, for any non-null reference values {@code x} and * {@code y}, this method returns {@code true} if and only * if {@code x} and {@code y} refer to the same object * ({@code x == y} has the value {@code true}). * <p> * Note that it is generally necessary to override the {@code hashCode} * method whenever this method is overridden, so as to maintain the * general contract for the {@code hashCode} method, which states * that equal objects must have equal hash codes. * * @param obj the reference object with which to compare. * @return {@code true} if this object is the same as the obj * argument; {@code false} otherwise. * @see #hashCode() * @see java.util.HashMap */ public boolean equals(Object obj) { return (this == obj); }
4：為了認清==比較的本質，我又多實驗了一把，如下所示
public class EquStringTestMain { public static void main(String[]args){ String _String1 = "godtrue"; String _String2 = "godtrue"; System.out.println("_String1 value is : "+_String1+"\n_String2 value is : "+_String2); System.out.println("_String1 identityHashCode is :"+System.identityHashCode(_String1)+"\n_String2 identityHashCode is :"+System.identityHashCode(_String2)); System.out.println("_String1==_String2 is : "+(_String1==_String2)+"\n"); String _String3 = new String("godtrue"); String _String4 = new String("godtrue"); System.out.println("_String3 value is : "+_String3+"\n_String4 value is : "+_String4); System.out.println("_String3 identityHashCode is :"+System.identityHashCode(_String3)+"\n_String4 identityHashCode is :"+System.identityHashCode(_String4)); System.out.println("_String3==_String4 is : "+(_String3==_String4)+"\n"); String _String5 = _String1; String _String6 = _String1; System.out.println("_String5 value is : "+_String5+"\n_String6 value is : "+_String6); System.out.println("_String5 identityHashCode is :"+System.identityHashCode(_String5)+"\n_String6 identityHashCode is :"+System.identityHashCode(_String6)); System.out.println("_String5==_String6 is : "+(_String5==_String6)+"\n"); String _String7 = _String3; String _String8 = _String3; System.out.println("_String7 value is : "+_String7+"\n_String8 value is : "+_String8); System.out.println("_String7 identityHashCode is :"+System.identityHashCode(_String7)+"\n_String8 identityHashCode is :"+System.identityHashCode(_String8)); System.out.println("_String7==_String8 is : "+(_String7==_String8)+"\n"); String _String9 = _String1; String _String0 = _String3; System.out.println("_String9 value is : "+_String9+"\n_String0 value is : "+_String0); System.out.println("_String9 identityHashCode is :"+System.identityHashCode(_String9)+"\n_String0 identityHashCode is :"+System.identityHashCode(_String0)); System.out.println("_String9==_String0 is : "+(_String9==_String0)+"\n"); } }
上例運行的結果，如下所示（每次運行identityHashCode都不同的喲）：
"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7534 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.EquStringTestMain _String1 value is : godtrue _String2 value is : godtrue _String1 identityHashCode is :1748462787 _String2 identityHashCode is :1748462787 _String1==_String2 is : true _String3 value is : godtrue _String4 value is : godtrue _String3 identityHashCode is :247835561 _String4 identityHashCode is :1401482194 _String3==_String4 is : false _String5 value is : godtrue _String6 value is : godtrue _String5 identityHashCode is :1748462787 _String6 identityHashCode is :1748462787 _String5==_String6 is : true _String7 value is : godtrue _String8 value is : godtrue _String7 identityHashCode is :247835561 _String8 identityHashCode is :247835561 _String7==_String8 is : true _String9 value is : godtrue _String0 value is : godtrue _String9 identityHashCode is :1748462787 _String0 identityHashCode is :247835561 _String9==_String0 is : false
5：String類相對特殊一點，再來一個栗子嘗嘗
public class EquTestMain { public static void main(String[]args){ int int1 = 9; int int2 = 9; System.out.println("int1 value is : "+int1+"\nint2 value is : "+int2); System.out.println("int1 identityHashCode is :"+System.identityHashCode(int1)+"\nint2 identityHashCode is :"+System.identityHashCode(int2)); System.out.println("int1==int2 is : "+(int1==int2)+"\n"); Integer _Integer1 = 9; Integer _Integer2 = 9; System.out.println("_Integer1 value is : "+_Integer1+"\n_Integer2 value is : "+_Integer2); System.out.println("_Integer1 identityHashCode is :"+System.identityHashCode(_Integer1)+"\n_Integer2 identityHashCode is :"+System.identityHashCode(_Integer2)); System.out.println("_Integer1==_Integer2 is : "+(_Integer1==_Integer2)+"\n"); Integer _Integer3 = new Integer(9); Integer _Integer4 = new Integer(9); System.out.println("_Integer3 value is : "+_Integer3+"\n_Integer4 value is : "+_Integer4); System.out.println("_Integer3 identityHashCode is :"+System.identityHashCode(_Integer3)+"\n_Integer4 identityHashCode is :"+System.identityHashCode(_Integer4)); System.out.println("_Integer3==_Integer4 is : "+(_Integer3==_Integer4)+"\n"); Integer _Integer5 = int1; Integer _Integer6 = int1; System.out.println("_Integer5 value is : "+_Integer5+"\n_Integer6 value is : "+_Integer6); System.out.println("_Integer5 identityHashCode is :"+System.identityHashCode(_Integer5)+"\n_Integer6 identityHashCode is :"+System.identityHashCode(_Integer6)); System.out.println("_Integer5==_Integer6 is : "+(_Integer5==_Integer6)+"\n"); Integer _Integer7 = _Integer1; Integer _Integer8 = _Integer1; System.out.println("_Integer7 value is : "+_Integer7+"\n_Integer8 value is : "+_Integer8); System.out.println("_Integer7 identityHashCode is :"+System.identityHashCode(_Integer7)+"\n_Integer8 identityHashCode is :"+System.identityHashCode(_Integer8)); System.out.println("_Integer7==_Integer8 is : "+(_Integer7==_Integer8)+"\n"); Integer _Integer9 = _Integer3; Integer _Integer0 = _Integer3; System.out.println("_Integer9 value is : "+_Integer9+"\n_Integer0 value is : "+_Integer0); System.out.println("_Integer9 identityHashCode is :"+System.identityHashCode(_Integer9)+"\n_Integer0 identityHashCode is :"+System.identityHashCode(_Integer0)); System.out.println("_Integer9==_Integer0 is : "+(_Integer9==_Integer0)+"\n"); } }
上例運行的結果，如下所示（每次運行identityHashCode都不同的喲）：
"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7536 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.EquTestMain int1 value is : 9 int2 value is : 9 int1 identityHashCode is :1401482194 int2 identityHashCode is :1401482194 int1==int2 is : true _Integer1 value is : 9 _Integer2 value is : 9 _Integer1 identityHashCode is :1401482194 _Integer2 identityHashCode is :1401482194 _Integer1==_Integer2 is : true _Integer3 value is : 9 _Integer4 value is : 9 _Integer3 identityHashCode is :1110594262 _Integer4 identityHashCode is :1977385357 _Integer3==_Integer4 is : false _Integer5 value is : 9 _Integer6 value is : 9 _Integer5 identityHashCode is :1401482194 _Integer6 identityHashCode is :1401482194 _Integer5==_Integer6 is : true _Integer7 value is : 9 _Integer8 value is : 9 _Integer7 identityHashCode is :1401482194 _Integer8 identityHashCode is :1401482194 _Integer7==_Integer8 is : true _Integer9 value is : 9 _Integer0 value is : 9 _Integer9 identityHashCode is :1110594262 _Integer0 identityHashCode is :1110594262 _Integer9==_Integer0 is : true
6：如上的代碼，如果看完了，也理解了，基本上結論也出來了。（建議先看下hashCode和identityHashCode 的關系）
6-1：==是比較運算符，當是基本數據類型時，比較的是變量的值，當是其他類型的對象時，用它比較的是兩個對象的引用地址值是否相等
6-2：equals是一個方法，如果對應的類沒有重現Object類的equals()方法，則和==是一樣的作用
6-3：identityHashCode更能間接的代表一個對象的引用地址值
6-4：如果一個類沒有重寫Object類的hashCode方法那么hashCode和identityHashCode是一致的
6-5：hashCode相等時使用==比較，不一定是true
6-6：identityHashCode相等時使用==比較，一定是true
6-7：hashCode相等時使用equals比較，也不一定是true（通常可認為是true，我們自己可以隨便定義的）
6-8：我們通常關心的是對象值是否相當（一個值或者多個值），所以對象之間的比較一定要使用equals方法，而非==
7：參看博文如下（觀點一致，只是解釋的思路或者栗子不同）
Java ==和equals()的區別【詳細版】
Java中equals和==的區別
What is the difference between == vs equals() in Java?
java：String使用equals和==比較的區別
java中equals和==的區別
淺談Java中的equals和==