Java で文字列操作する関数・メソッドの一覧

String

先頭・末尾の文字列検索 (startsWith, endsWith)

String str = "abcdef";
System.out.println(str.startsWith("abc")); // => true
System.out.println(str.endsWith("ghi"));   // => false

指定文字列で連結 (String.join)

String[] elements = {"abc", "def", "ghi"};
System.out.println(String.join(",", elements)); // => abc,def,ghi

前後の空白を削除 (trim)

String str = "    abc\n\t  ";
System.out.println(str.trim()); // => abc

部分文字列 (substring)

String input = "abcdefghi";
System.out.println(input.substring(5,7)); // => fg

文字列の検索 (indexOf)

String input = "abcdefghi";
System.out.println(input.indexOf("def")); // => 3
System.out.println(input.indexOf("jkl")); // => -1

文字列の置換 (replace, replaceAll, replaceFirst)

正規表現が入力可能

String input = "abc, def, ghi";
System.out.println(input.replaceAll(",\s*", ";"));
// => abc;def;ghi

文字列の分割 (split)

正規表現が入力可能

String input = "abc, def, ghi,123";
String[] res = input.split(",\s*");
for (String str : res) {
  System.out.println(str);
}
// => abc
// => def
// => ghi
// => 123

文字列の比較 (equals, compareTo)

String input1 = "AAA";
String input2 = "BBB";
System.out.println(input1.equals(input2));    // => false
System.out.println(input1.compareTo(input2)); // => -1

String input3 = "AAA";
System.out.println(input1.equals(input3));    // => true
System.out.println(input1.compareTo(input3)); // => 0

文字列バッファ (StringBuilder)

StringBuilder sb = new StringBuilder();
sb.append("hello, ");
sb.append("world!");
System.out.println(sb); // => hello, world!

エンコーディング

String str = "こんにちは";
try {
    byte[] bytes = str.getBytes("SJIS");
    System.out.println(Arrays.toString(bytes));
    // => [-126, -79, -126, -15, -126, -55, -126, -65, -126, -51]
    
    String newstr = new String(bytes, "SJIS");
    System.out.println(newstr);
    // => こんにちは

} catch (UnsupportedEncodingException e) {
    e.printStackTrace();
}

正規表現で完全マッチ判定

String pattern = "\\d{3}-\\d{4}";
System.out.println("123-4321".matches(pattern));  // => true
System.out.println("345-67890".matches(pattern)); // => false

正規表現で検索とグルーピング

String input = "038-123-4567";
Pattern pattern = Pattern.compile("(\\d{3})-(\\d{3})-(\\d{4})");
Matcher m = pattern.matcher(input);
if (m.find()) {
    System.out.println(m.group(1)); // => 038
    System.out.println(m.group(2)); // => 123
    System.out.println(m.group(3)); // => 4567
}

正規表現で置換

String input = "<1>2<3>4";
System.out.println(input.replaceAll("<(\\d)>", "{$1}")); // => {1}2{3}4