8341566: Add Reader.of(CharSequence)

Reviewed-by: rriggs, jpai, liach, alanb

Author: mkarg

src/java.base/share/classes/java/io/Reader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 2024, 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
@@ -140,6 +140,119 @@ public void close() {
         };
     }
 
+    /**
+     * Returns a {@code Reader} that reads characters from a
+     * {@code CharSequence}. The reader is initially open and reading starts at
+     * the first character in the sequence.
+     *
+     * <p> The returned reader supports the {@link #mark mark()} and
+     * {@link #reset reset()} operations.
+     *
+     * <p> The resulting reader is not safe for use by multiple
+     * concurrent threads. If the reader is to be used by more than one
+     * thread it should be controlled by appropriate synchronization.
+     *
+     * <p> If the sequence changes while the reader is open, e.g. the length
+     * changes, the behavior is undefined.
+     *
+     * @param cs {@code CharSequence} providing the character stream.
+     * @return a {@code Reader} which reads characters from {@code cs}
+     * @throws NullPointerException if {@code cs} is {@code null}
+     *
+     * @since 24
+     */
+    public static Reader of(final CharSequence cs) {
+        Objects.requireNonNull(cs);
+
+        return new Reader() {
+            private boolean isClosed;
+            private int next = 0;
+            private int mark = 0;
+
+            /** Check to make sure that the stream has not been closed */
+            private void ensureOpen() throws IOException {
+                if (isClosed)
+                    throw new IOException("Stream closed");
+            }
+
+            @Override
+            public int read() throws IOException {
+                ensureOpen();
+                if (next >= cs.length())
+                    return -1;
+                return cs.charAt(next++);
+            }
+
+            @Override
+            public int read(char[] cbuf, int off, int len) throws IOException {
+                ensureOpen();
+                Objects.checkFromIndexSize(off, len, cbuf.length);
+                if (len == 0) {
+                    return 0;
+                }
+                int length = cs.length();
+                if (next >= length)
+                    return -1;
+                int n = Math.min(length - next, len);
+                switch (cs) {
+                    case String s -> s.getChars(next, next + n, cbuf, off);
+                    case StringBuilder sb -> sb.getChars(next, next + n, cbuf, off);
+                    case StringBuffer sb -> sb.getChars(next, next + n, cbuf, off);
+                    case CharBuffer cb -> cb.get(next, cbuf, off, n);
+                    default -> {
+                        for (int i = 0; i < n; i++)
+                            cbuf[off + i] = cs.charAt(next + i);
+                    }
+                }
+                next += n;
+                return n;
+            }
+
+            @Override
+            public long skip(long n) throws IOException {
+                ensureOpen();
+                if (next >= cs.length())
+                    return 0;
+                // Bound skip by beginning and end of the source
+                long r = Math.min(cs.length() - next, n);
+                r = Math.max(-next, r);
+                next += (int)r;
+                return r;
+            }
+
+            @Override
+            public boolean ready() throws IOException {
+                ensureOpen();
+                return true;
+            }
+
+            @Override
+            public boolean markSupported() {
+                return true;
+            }
+
+            @Override
+            public void mark(int readAheadLimit) throws IOException {
+                if (readAheadLimit < 0){
+                    throw new IllegalArgumentException("Read-ahead limit < 0");
+                }
+                ensureOpen();
+                mark = next;
+            }
+
+            @Override
+            public void reset() throws IOException {
+                ensureOpen();
+                next = mark;
+            }
+
+            @Override
+            public void close() {
+                isClosed = true;
+            }
+        };
+    }
+
     /**
      * The object used to synchronize operations on this stream.  For
      * efficiency, a character-stream object may use an object other than

src/java.base/share/classes/java/io/StringReader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 2024, 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
@@ -30,31 +30,25 @@
 /**
  * A character stream whose source is a string.
  *
+ * @apiNote
+ * {@link Reader#of(CharSequence)} provides a method to read from any
+ * {@link CharSequence} that may be more efficient than {@code StringReader}.
+ *
  * @author      Mark Reinhold
  * @since       1.1
  */
 
 public class StringReader extends Reader {
 
-    private final int length;
-    private String str;
-    private int next = 0;
-    private int mark = 0;
+    private final Reader r;
 
     /**
      * Creates a new string reader.
      *
      * @param s  String providing the character stream.
      */
     public StringReader(String s) {
-        this.length = s.length();
-        this.str = s;
-    }
-
-    /** Check to make sure that the stream has not been closed */
-    private void ensureOpen() throws IOException {
-        if (str == null)
-            throw new IOException("Stream closed");
+        r = Reader.of(s);
     }
 
     /**
@@ -67,10 +61,7 @@ private void ensureOpen() throws IOException {
      */
     public int read() throws IOException {
         synchronized (lock) {
-            ensureOpen();
-            if (next >= length)
-                return -1;
-            return str.charAt(next++);
+            return r.read();
         }
     }
 
@@ -94,17 +85,7 @@ public int read() throws IOException {
      */
     public int read(char[] cbuf, int off, int len) throws IOException {
         synchronized (lock) {
-            ensureOpen();
-            Objects.checkFromIndexSize(off, len, cbuf.length);
-            if (len == 0) {
-                return 0;
-            }
-            if (next >= length)
-                return -1;
-            int n = Math.min(length - next, len);
-            str.getChars(next, next + n, cbuf, off);
-            next += n;
-            return n;
+            return r.read(cbuf, off, len);
         }
     }
 
@@ -130,14 +111,7 @@ public int read(char[] cbuf, int off, int len) throws IOException {
      */
     public long skip(long n) throws IOException {
         synchronized (lock) {
-            ensureOpen();
-            if (next >= length)
-                return 0;
-            // Bound skip by beginning and end of the source
-            long r = Math.min(length - next, n);
-            r = Math.max(-next, r);
-            next += (int)r;
-            return r;
+            return r.skip(n);
         }
     }
 
@@ -150,8 +124,7 @@ public long skip(long n) throws IOException {
      */
     public boolean ready() throws IOException {
         synchronized (lock) {
-            ensureOpen();
-            return true;
+            return r.ready();
         }
     }
 
@@ -176,12 +149,8 @@ public boolean markSupported() {
      * @throws     IOException  If an I/O error occurs
      */
     public void mark(int readAheadLimit) throws IOException {
-        if (readAheadLimit < 0){
-            throw new IllegalArgumentException("Read-ahead limit < 0");
-        }
         synchronized (lock) {
-            ensureOpen();
-            mark = next;
+            r.mark(readAheadLimit);
         }
     }
 
@@ -193,8 +162,7 @@ public void mark(int readAheadLimit) throws IOException {
      */
     public void reset() throws IOException {
         synchronized (lock) {
-            ensureOpen();
-            next = mark;
+            r.reset();
         }
     }
 
@@ -207,7 +175,11 @@ public void reset() throws IOException {
      */
     public void close() {
         synchronized (lock) {
-            str = null;
+            try {
+                r.close();
+            } catch (IOException e) {
+                throw new UncheckedIOException(e);
+            }
         }
     }
 }