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8356995: Provide default methods min(T, T) and max(T, T) in Comparator interface

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Reviewed-by: rriggs, smarks
This commit is contained in:
Tagir F. Valeev 2025-09-19 07:19:03 +00:00
parent 898fcff037
commit 48d394a245
2 changed files with 137 additions and 2 deletions
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49
src/java.base/share/classes/java/util/Comparator.java
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@ -1,5 +1,5 @@
/*
* Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 1997, 2025, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
@ -30,7 +30,6 @@ import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;
/**
* A comparison function, which imposes a <i>total ordering</i> on
@ -189,6 +188,52 @@ public interface Comparator<T> {
return Collections.reverseOrder(this);
}
/**
* Returns the greater of two values according to this comparator.
* If the arguments are equal with respect to this comparator,
* the {@code o1} argument is returned.
*
* @implSpec This default implementation behaves as if
* {@code compare(o1, o2) >= 0 ? o1 : o2}.
*
* @param o1 an argument.
* @param o2 another argument.
* @param <U> the type of the arguments and the result.
* @return the larger of {@code o1} and {@code o2} according to this comparator.
* @throws NullPointerException if an argument is null and this
* comparator does not permit null arguments
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*
* @since 26
*/
default <U extends T> U max(U o1, U o2) {
return compare(o1, o2) >= 0 ? o1 : o2;
}
/**
* Returns the smaller of two values according to this comparator.
* If the arguments are equal with respect to this comparator,
* the {@code o1} argument is returned.
*
* @implSpec This default implementation behaves as if
* {@code compare(o1, o2) <= 0 ? o1 : o2}.
*
* @param o1 an argument.
* @param o2 another argument.
* @param <U> the type of the arguments and the result.
* @return the smaller of {@code o1} and {@code o2} according to this comparator.
* @throws NullPointerException if an argument is null and this
* comparator does not permit null arguments
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*
* @since 26
*/
default <U extends T> U min(U o1, U o2) {
return compare(o1, o2) <= 0 ? o1 : o2;
}
/**
* Returns a lexicographic-order comparator with another comparator.
* If this {@code Comparator} considers two elements equal, i.e.

90
test/jdk/java/util/Comparator/MinMaxTest.java Normal file
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@ -0,0 +1,90 @@
/*
* Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/**
* @test
* @bug 8356995
* @summary Comparator min/max method tests
* @run junit MinMaxTest
*/
import org.junit.jupiter.api.Test;
import java.util.Comparator;
import static org.junit.jupiter.api.Assertions.*;
public class MinMaxTest {
@Test
void testMin() {
Comparator<String> c = Comparator.naturalOrder();
assertEquals("a", c.min("a", "b"));
assertEquals("a", c.min("b", "a"));
}
@Test
void testMax() {
Comparator<String> c = Comparator.naturalOrder();
assertEquals("b", c.max("a", "b"));
assertEquals("b", c.max("b", "a"));
}
@Test
void testThrowsNPE() {
Comparator<String> c = Comparator.naturalOrder();
assertThrows(NullPointerException.class, () -> c.min(null, "a"));
assertThrows(NullPointerException.class, () -> c.min("a", null));
assertThrows(NullPointerException.class, () -> c.max(null, "a"));
assertThrows(NullPointerException.class, () -> c.max("a", null));
}
@Test
void testThrowsCCE() {
@SuppressWarnings("unchecked")
Comparator<Object> c = (Comparator<Object>) (Comparator<?>)Comparator.naturalOrder();
assertThrows(ClassCastException.class, () -> c.min(1, "a"));
assertThrows(ClassCastException.class, () -> c.min("a", 1));
assertThrows(ClassCastException.class, () -> c.max(1, "a"));
assertThrows(ClassCastException.class, () -> c.max("a", 1));
}
@Test
void testEqualReturnFirst() {
Comparator<Object> allEqual = (_, _) -> 0;
Object o1 = new Object();
Object o2 = new Object();
assertSame(o1, allEqual.min(o1, o2));
assertSame(o1, allEqual.max(o1, o2));
}
@Test
void testComparatorSubtype() {
Comparator<CharSequence> byLength = Comparator.comparing(CharSequence::length);
String s1 = "long_string";
String s2 = "short";
String min = byLength.min(s1, s2);
String max = byLength.max(s1, s2);
assertEquals(s1, max);
assertEquals(s2, min);
}
}