[Java] Object.hashCode JavaDoc - History

Java 1.3.1

hashCode

public int hashCode()Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Returns:a hash code value for this object.See Also:equals(java.lang.Object), Hashtable

Java 1.4.2

Java 1.3.1 과 동일

hashCode

public int hashCode()Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Returns:a hash code value for this object.See Also:equals(java.lang.Object), Hashtable

Java 8

link: https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode--

Java 1.3.1 과 동일

Java 9

https://docs.oracle.com/javase/9/docs/api/java/lang/Object.html#hashCode--

hashCode

public int hashCode​()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (The hashCode may or may not be implemented as some function of an object's memory address at some point in time.)

Returns:

a hash code value for this object.

See Also:

equals(java.lang.Object), System.identityHashCode(java.lang.Object)

Java 10

https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#hashCode()

Java 9과 동일

Java 11

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#hashCode()

Java 9과 동일

Java 12

https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/Object.html#hashCode()

Implementaion Requirements 로 분리

hashCode

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

Implementation Requirements:

As far as is reasonably practical, the hashCode method defined by class Object returns distinct integers for distinct objects.

Returns:

a hash code value for this object.

See Also:

equals(java.lang.Object), System.identityHashCode(java.lang.Object)

Java 13

https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/Object.html#hashCode()

Java 12와 동일