【Java入門】要素の最初の値を取得する方法（ストリームAPI）

2022/2/23 Java

JavaでストリームAPIを使用して要素の最初の値を取得する方法について記載します。

目次

要素の最初の値を取得する方法

findFirst メソッドを使用します。

構文

findFirst()

戻り値

Optional<T>

1. 最初の値を取得

実行例

// 最初の値を取得
List<String> list = Arrays.asList("aa","ab","bc");

Optional<String> str1 = list.stream().findFirst();
if( str1.isPresent() ) {
    System.out.println(str1.get()); // aa
}

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// 最初の値を取得

List<String> list = Arrays.asList("aa","ab","bc");

Optional<String> str1 = list.stream().findFirst();

if( str1.isPresent() ) {

System.out.println(str1.get()); // aa

}

2. 条件に一致する最初の要素を取得

filterメソッドと組み合わせて、指定した条件に一致する最初の値を取得します。

実行例

// 文字列に "b" が含まれる最初の値を取得
List<String> list = Arrays.asList("aa","ab","bc");

Optional<String> str2 = list.stream().filter( s -> s.contains("b")).findFirst();
if( str2.isPresent() ) {
    System.out.println(str2.get()); // ab
}

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// 文字列に "b" が含まれる最初の値を取得

List<String> list = Arrays.asList("aa","ab","bc");

Optional<String> str2 = list.stream().filter( s -> s.contains("b")).findFirst();

if( str2.isPresent() ) {

System.out.println(str2.get()); // ab

}

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