The Comparable.java Java example source code
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package java.lang;
import java.util.*;
/**
* This interface imposes a total ordering on the objects of each class that
* implements it. This ordering is referred to as the class's <i>natural
* ordering</i>, and the class's compareTo method is referred to as
* its <i>natural comparison method.
*
* Lists (and arrays) of objects that implement this interface can be sorted
* automatically by {@link Collections#sort(List) Collections.sort} (and
* {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this
* interface can be used as keys in a {@linkplain SortedMap sorted map} or as
* elements in a {@linkplain SortedSet sorted set}, without the need to
* specify a {@linkplain Comparator comparator}.<p>
*
* The natural ordering for a class <tt>C is said to be consistent
* with equals</i> if and only if e1.compareTo(e2) == 0 has
* the same boolean value as <tt>e1.equals(e2) for every
* <tt>e1 and e2 of class C. Note that null
* is not an instance of any class, and <tt>e.compareTo(null) should
* throw a <tt>NullPointerException even though e.equals(null)
* returns <tt>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 <tt>equals
* method.<p>
*
* For example, if one adds two keys <tt>a and b such that
* {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted
* set that does not use an explicit comparator, the second <tt>add
* operation returns false (and the size of the sorted set does not increase)
* because <tt>a and b are equivalent from the sorted set's
* perspective.<p>
*
* Virtually all Java core classes that implement <tt>Comparable have natural
* orderings that are consistent with equals. One exception is
* <tt>java.math.BigDecimal, whose natural ordering equates
* <tt>BigDecimal objects with equal values and different precisions
* (such as 4.0 and 4.00).<p>
*
* For the mathematically inclined, the <i>relation that defines
* the natural ordering on a given class C is:<pre>
* {(x, y) such that x.compareTo(y) <= 0}.
* </pre> The quotient for this total order is:
* {(x, y) such that x.compareTo(y) == 0}.
* </pre>
*
* It follows immediately from the contract for <tt>compareTo that the
* quotient is an <i>equivalence relation on C, and that the
* natural ordering is a <i>total order on C. When we say that a
* class's natural ordering is <i>consistent with equals, we mean that the
* quotient for the natural ordering is the equivalence relation defined by
* the class's {@link Object#equals(Object) equals(Object)} method:<pre>
* {(x, y) such that x.equals(y)}. </pre>
*
* This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <T> the type of objects that this object may be compared to
*
* @author Josh Bloch
* @see java.util.Comparator
* @since 1.2
*/
public interface Comparable<T> {
/**
* 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.
*
* <p>The implementor must ensure sgn(x.compareTo(y)) ==
* -sgn(y.compareTo(x))</tt> for all x and y. (This
* implies that <tt>x.compareTo(y) must throw an exception iff
* <tt>y.compareTo(x) throws an exception.)
*
* <p>The implementor must also ensure that the relation is transitive:
* <tt>(x.compareTo(y)>0 && y.compareTo(z)>0) implies
* <tt>x.compareTo(z)>0.
*
* <p>Finally, the implementor must ensure that x.compareTo(y)==0
* implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for
* all <tt>z.
*
* <p>It is strongly recommended, but not strictly required that
* <tt>(x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any
* class that implements the <tt>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."
*
* <p>In the foregoing description, the notation
* <tt>sgn(expression) designates the mathematical
* <i>signum function, which is defined to return one of -1,
* <tt>0, or 1 according to whether the value of
* <i>expression is negative, zero or positive.
*
* @param o the object to be compared.
* @return a negative integer, zero, or a positive integer as this object
* is less than, equal to, or greater than the specified object.
*
* @throws NullPointerException if the specified object is null
* @throws ClassCastException if the specified object's type prevents it
* from being compared to this object.
*/
public int compareTo(T o);
}
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