8203442: String::transform

Reviewed-by: smarks, sherman, alanb, darcy, forax, rriggs, scolebourne, dholmes, plevart

src/java.base/share/classes/java/lang/String.java

@@ -37,6 +37,7 @@ import java.util.Locale;
 import java.util.Objects;
 import java.util.Spliterator;
 import java.util.StringJoiner;
+import java.util.function.Function;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;
@@ -2972,6 +2973,25 @@ public final class String
         return indent(indent > n ? Integer.MIN_VALUE : indent, true);
     }
 
+    /**
+     * This method allows the application of a function to {@code this}
+     * string. The function should expect a single String argument
+     * and produce an {@code R} result.
+     *
+     * @param f    functional interface to a apply
+     *
+     * @param <R>  class of the result
+     *
+     * @return     the result of applying the function to this string
+     *
+     * @see java.util.function.Function
+     *
+     * @since 12
+     */
+    public <R> R transform(Function<? super String, ? extends R> f) {
+        return f.apply(this);
+    }
+
     /**
      * This object (which is already a string!) is itself returned.
      *

test/jdk/java/lang/String/Transform.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2018, 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
+ * @summary Unit tests for String#transform(Function<String, R> f)
+ * @run main Transform
+ */
+
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+public class Transform {
+    public static void main(String[] args) {
+        test1();
+    }
+
+    /*
+     * Test String#transform(Function<? super String, ? extends R> f) functionality.
+     */
+    static void test1() {
+        simpleTransform("toUpperCase", "abc", s -> s.toUpperCase());
+        simpleTransform("toLowerCase", "ABC", s -> s.toLowerCase());
+        simpleTransform("substring", "John Smith", s -> s.substring(0, 4));
+
+        String multiline = "    This is line one\n" +
+                           "        This is line two\n" +
+                           "            This is line three\n";
+        String expected = "This is line one!\n" +
+                          "    This is line two!\n" +
+                          "        This is line three!\n";
+        check("multiline", multiline.transform(string -> {
+            return string.lines()
+                         .map(s -> s.transform(t -> t.substring(4) + "!"))
+                         .collect(Collectors.joining("\n", "", "\n"));
+        }), expected);
+    }
+
+    static void simpleTransform(String test, String s, Function<String, String> f) {
+        check(test, s.transform(f), f.apply(s));
+    }
+
+    static void check(String test, Object output, Object expected) {
+        if (output != expected && (output == null || !output.equals(expected))) {
+            System.err.println("Testing " + test + ": unexpected result");
+            System.err.println("Output:");
+            System.err.println(output);
+            System.err.println("Expected:");
+            System.err.println(expected);
+            throw new RuntimeException();
+        }
+    }
+}