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@nanaco

Java Integerの比較(==)が正しく動作しない

More than 1 year has passed since last update.

現象

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in); // 「2 10000000 10000000」が入ってくる
        int n = sc.nextInt();
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < n; i++) list.add(sc.nextInt());

        for (int i = 0; i < n-1; i++) {
            if (list.get(i) == list.get(i+1)) {
                System.out.println("等しい");
                return;
            }
        }
        System.out.println("等しくない");
    }
}

list.get(i)には10000000、list.get(i+1)にもおなじ10000000のはずなのに結果が「等しくない」が出力されてしまいました。なぜ??

プリミティブ型は==、オブジェクト型はequals

Integerはオブジェクト型のため、==で比較する場合、同一のインスタンスかどうかを確認することになるため、「等しくない」になってしまうようです。

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in); // 「2 10000000 10000000」が入ってくる
        int n = sc.nextInt();
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < n; i++) list.add(sc.nextInt());

        for (int i = 0; i < n-1; i++) {
            if (list.get(i).equals(list.get(i+1))) {
                System.out.println("等しい");
                return;
            }
        }
        System.out.println("等しくない");
    }
}

// 結果
// 等しい

結論、数を比較する際は、プリミティブ型は==、オブジェクト型はequalsを使いように注意しないとでしたー。

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