Java compareto with null
This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class’s natural ordering, and the class’s compareTo method is referred to as its natural comparison method. Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort ). Objects that implement this interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparator. The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C. Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false. It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave «strangely» when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method. For example, if one adds two keys a and b such that (!a.equals(b) && a.compareTo(b) == 0) to a sorted set that does not use an explicit comparator, the second add operation returns false (and the size of the sorted set does not increase) because a and b are equivalent from the sorted set’s perspective. Virtually all Java core classes that implement Comparable have natural orderings that are consistent with equals. One exception is java.math.BigDecimal, whose natural ordering equates BigDecimal objects with equal values and different precisions (such as 4.0 and 4.00). For the mathematically inclined, the relation that defines the natural ordering on a given class C is:
It follows immediately from the contract for compareTo that the quotient is an equivalence relation on C, and that the natural ordering is a total order on C. When we say that a class’s natural ordering is consistent with equals, we mean that the quotient for the natural ordering is the equivalence relation defined by the class’s equals(Object) method:
Method Summary
Method Detail
compareTo
Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object. The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.) The implementor must also ensure that the relation is transitive: (x.compareTo(y)>0 && y.compareTo(z)>0) implies x.compareTo(z)>0. Finally, the implementor must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z. It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is «Note: this class has a natural ordering that is inconsistent with equals.» In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.
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Interface Comparable
This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class’s natural ordering, and the class’s compareTo method is referred to as its natural comparison method.
Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort ). Objects that implement this interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparator.
The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C . Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false .
It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave «strangely» when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.
For example, if one adds two keys a and b such that (!a.equals(b) && a.compareTo(b) == 0) to a sorted set that does not use an explicit comparator, the second add operation returns false (and the size of the sorted set does not increase) because a and b are equivalent from the sorted set’s perspective.
Virtually all Java core classes that implement Comparable have natural orderings that are consistent with equals. One exception is BigDecimal , whose natural ordering equates BigDecimal objects with equal numerical values and different representations (such as 4.0 and 4.00). For BigDecimal.equals() to return true, the representation and numerical value of the two BigDecimal objects must be the same.
For the mathematically inclined, the relation that defines the natural ordering on a given class C is:
It follows immediately from the contract for compareTo that the quotient is an equivalence relation on C , and that the natural ordering is a total order on C . When we say that a class’s natural ordering is consistent with equals, we mean that the quotient for the natural ordering is the equivalence relation defined by the class’s equals(Object) method:
In other words, when a class’s natural ordering is consistent with equals, the equivalence classes defined by the equivalence relation of the equals method and the equivalence classes defined by the quotient of the compareTo method are the same.
This interface is a member of the Java Collections Framework.
Java compareto with null
This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class’s natural ordering, and the class’s compareTo method is referred to as its natural comparison method. Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort ). Objects that implement this interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparator. The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C . Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false . It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave «strangely» when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method. For example, if one adds two keys a and b such that (!a.equals(b) && a.compareTo(b) == 0) to a sorted set that does not use an explicit comparator, the second add operation returns false (and the size of the sorted set does not increase) because a and b are equivalent from the sorted set’s perspective. Virtually all Java core classes that implement Comparable have natural orderings that are consistent with equals. One exception is java.math.BigDecimal , whose natural ordering equates BigDecimal objects with equal values and different precisions (such as 4.0 and 4.00). For the mathematically inclined, the relation that defines the natural ordering on a given class C is:
It follows immediately from the contract for compareTo that the quotient is an equivalence relation on C , and that the natural ordering is a total order on C . When we say that a class’s natural ordering is consistent with equals, we mean that the quotient for the natural ordering is the equivalence relation defined by the class’s equals(Object) method:
Method Summary
Method Detail
compareTo
Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object. The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y . (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.) The implementor must also ensure that the relation is transitive: (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0 . Finally, the implementor must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)) , for all z . It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)) . Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is «Note: this class has a natural ordering that is inconsistent with equals.» In the foregoing description, the notation sgn( expression ) designates the mathematical signum function, which is defined to return one of -1 , 0 , or 1 according to whether the value of expression is negative, zero, or positive, respectively.
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For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples.
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Copyright © 1993, 2023, Oracle and/or its affiliates, 500 Oracle Parkway, Redwood Shores, CA 94065 USA.
All rights reserved. Use is subject to license terms and the documentation redistribution policy.
Comparing Strings with (possible) null values in java?
Strings in Java represents an array of characters. They are represented by the String class.
Using compareTo() method
The compareTo() method of the String class two Strings (char by char) it also accepts null values. This method returns an integer representing the result, if the value of the obtained integer is −
- 0: Given two Strings are equal or, null.
- 1 or less: The current String preceeds the argument.
- 1 or more: The current String succeeds the argument.
Example
import java.util.Scanner; public class CompringStrings < public static void main(String args[]) < Scanner sc = new Scanner(System.in); System.out.println("Enter your first string value: "); String str1 = sc.next(); System.out.println("Enter your second string value: "); String str2 = sc.next(); //Comparing two Strings int res = str1.compareTo(str2); System.out.println(res); if(res==0) < System.out.println("Both Strings are null or equal"); >else if(res<0)< System.out.println(""+str1+" preceeds "+str2+""); >else if(res>0) < System.out.println(""+str2+" preceeds "+str1+""); >> > Output
Enter your first string value: null Enter your second string value: null 0 Both Strings are null or equal
Output
Enter your first string value: mango Enter your second string value: apple -1 apple preceeds mango
Using equals() method of the Object class
In the same way the equals() method of the object class accepts two String values and returns a boolean value, which is true if both are equal (or, null) and false if not.
Example
import java.util.Scanner; public class CompringStrings < public static void main(String args[]) < Scanner sc = new Scanner(System.in); System.out.println("Enter your first string value: "); String str1 = sc.next(); System.out.println("Enter your second string value: "); String str2 = sc.next(); if(str1.equals(str2)) < System.out.println("Both Strings are null or equal"); >else < System.out.println("Both Strings are not equal"); >> > Output1
Enter your first string value: null Enter your second string value: null 0 Both Strings are null or equal