Java : Writer - API使用例

Writer (Java SE 21 & JDK 21) の使い方まとめです。
だいたいのメソッドを網羅済みです。
API仕様書のおともにどうぞ。

概要

文字ストリームに書き込むための抽象クラスです。

クラス構成

Writer は使い終わったら close が必要です。
try-with-resources文を使い、リソースリークが起きないようにしましょう。

※StringWriter など、一部 close しなくても問題ないクラスもあります。

final var path = Path.of("R:", "java-work", "aaa.txt");
System.out.println(path); // R:\java-work\aaa.txt
System.out.println(Files.exists(path)); // false

try (final Writer writer = Files.newBufferedWriter(path)) {

    writer.write("abcd");
    writer.write("XYZ");
}

final var str = Files.readString(path);
System.out.println(str); // abcdXYZ

フィールド

protected Object lock

このストリームに対する処理の同期に使用するオブジェクトです。

protectedです。
独自にサブクラスを作ることは少ないと思いますので、コード例は割愛します。

コンストラクタ

Writer ()

ライター自体でクリティカル・セクションが同期する文字ストリーム・ライターを新しく作成します。

protectedです。
独自にサブクラスを作ることは少ないと思いますので、コード例は割愛します。

Writer (Object lock)

指定されたオブジェクトでクリティカル・セクションが同期する文字ストリーム・ライターを新しく作成します。

protectedです。
独自にサブクラスを作ることは少ないと思いますので、コード例は割愛します。

メソッド

Writer append (char c)

指定された文字をこのライターに追加します。

try (final Writer writer = new StringWriter()) {

    System.out.println(writer.append('a')); // a
    System.out.println(writer.append('b')); // ab
    System.out.println(writer.append('X').append('Y').append('Z')); // abXYX
}

Writer append (CharSequence csq)

指定された文字シーケンスをこのライターに追加します。

try (final Writer writer = new StringWriter()) {
    final var ret = writer.append("abc").append("XYZ");
    System.out.println(ret); // abcXYZ
}

try (final Writer writer = new StringWriter()) {

    final CharSequence csq1 = "abc";

    System.out.println(writer.append(csq1)); // abc

    final var sb = new StringBuilder();
    final CharSequence csq2 = sb;

    sb.append("XY");
    System.out.println(writer.append(csq2)); // abcXY

    sb.append("Z");
    System.out.println(writer.append(csq2)); // abcXYXYZ
}

Writer append (CharSequence csq, int start, int end)

指定された文字シーケンスのサブシーケンスをこのライターに追加します。

final CharSequence csq = "abcd";

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 0, 1)); // a
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 0, 2)); // ab
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 0, 3)); // abc
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 0, 4)); // abcd
}

try (final Writer writer = new StringWriter()) {
    //writer.append(csq, 0, 5); // StringIndexOutOfBoundsException
}

final CharSequence csq = "abcd";

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 0, 4)); // abcd
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 1, 4)); // bcd
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 2, 4)); // cd
}

try (final Writer writer = new StringWriter()) {
    System.out.println(writer.append(csq, 3, 4)); // d
}

abstract void close ()

ストリームを最初にフラッシュして、閉じます。

final var path = Path.of("R:", "java-work", "aaa.txt");
System.out.println(path); // R:\java-work\aaa.txt

try (final Writer writer = Files.newBufferedWriter(path)) {
    writer.append("abcd");
}

final var str = Files.readString(path);
System.out.println(str); // abcd

// try-with-resources文を使わない例です。
final var path = Path.of("R:", "java-work", "aaa.txt");
System.out.println(path); // R:\java-work\aaa.txt

final Writer writer = Files.newBufferedWriter(path);
try {
    writer.append("abcd");
} finally {
    writer.close();
}

final var str = Files.readString(path);
System.out.println(str); // abcd

abstract void flush ()

ストリームをフラッシュします。

final var path = Path.of("R:", "java-work", "aaa.txt");
System.out.println(path); // R:\java-work\aaa.txt

try (final Writer writer = Files.newBufferedWriter(path)) {
    writer.append("abcd");
    System.out.println(Files.readString(path).isEmpty()); // true

    writer.flush();
    System.out.println(Files.readString(path)); // abcd
}

static Writer nullWriter ()

すべての文字を破棄する新規Writerを返します。

try (final var writer = Writer.nullWriter()) {
    // なにも起きません。
    writer.append("abc");
    writer.flush();
}

void write (char[] cbuf)

文字の配列を書き込みます。

try (final Writer writer = new StringWriter()) {

    final char[] cbuf1 = {'a', 'b', 'c'};

    writer.write(cbuf1);
    System.out.println(writer); // abc

    final char[] cbuf2 = {'X', 'Y', 'Z'};

    writer.write(cbuf2);
    System.out.println(writer); // abcXYZ
}

abstract void write (char[] cbuf, int off, int len)

文字の配列の一部を書き込みます。

final char[] cbuf = {'a', 'b', 'c', 'd'};

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 0, 1);
    System.out.println(writer); // a
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 0, 2);
    System.out.println(writer); // ab
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 0, 3);
    System.out.println(writer); // abc
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 0, 4);
    System.out.println(writer); // abcd
}

try (final Writer writer = new StringWriter()) {
    //writer.write(cbuf, 0, 5); // IndexOutOfBoundsException
}

final char[] cbuf = {'a', 'b', 'c', 'd'};

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 0, 4);
    System.out.println(writer); // abcd
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 1, 3);
    System.out.println(writer); // bcd
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 2, 2);
    System.out.println(writer); // cd
}

try (final Writer writer = new StringWriter()) {
    writer.write(cbuf, 3, 1);
    System.out.println(writer); // d
}

void write (int c)

単一の文字を書き込みます。

try (final Writer writer = new StringWriter()) {

    writer.write('a');
    System.out.println(writer); // a

    writer.write('b');
    System.out.println(writer); // ab

    writer.write('X');
    System.out.println(writer); // abX

    writer.write('Y');
    System.out.println(writer); // abXY

    writer.write('Z');
    System.out.println(writer); // abXYZ
}

void write (String str)

文字列を書き込みます。

try (final Writer writer = new StringWriter()) {

    writer.write("abc");
    System.out.println(writer); // abc

    writer.write("XYZ");
    System.out.println(writer); // abcXYZ
}

void write (String str, int off, int len)

文字列の一部を書き込みます。

final var str = "abcd";

try (final Writer writer = new StringWriter()) {
    writer.write(str, 0, 1);
    System.out.println(writer); // a
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 0, 2);
    System.out.println(writer); // ab
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 0, 3);
    System.out.println(writer); // abc
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 0, 4);
    System.out.println(writer); // abcd
}

try (final Writer writer = new StringWriter()) {
    //writer.write(str, 0, 5); // IndexOutOfBoundsException
}

final var str = "abcd";

try (final Writer writer = new StringWriter()) {
    writer.write(str, 0, 4);
    System.out.println(writer); // abcd
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 1, 3);
    System.out.println(writer); // bcd
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 2, 2);
    System.out.println(writer); // cd
}

try (final Writer writer = new StringWriter()) {
    writer.write(str, 3, 1);
    System.out.println(writer); // d
}

API 使用例

Java : Writer - API使用例

概要

フィールド

protected Object lock

コンストラクタ

Writer ()

Writer (Object lock)

メソッド

Writer append (char c)

Writer append (CharSequence csq)

Writer append (CharSequence csq, int start, int end)

abstract void close ()

abstract void flush ()

static Writer nullWriter ()

void write (char[] cbuf)

abstract void write (char[] cbuf, int off, int len)

void write (int c)

void write (String str)

void write (String str, int off, int len)

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