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Item 53: Use varargs judiciously

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53.可変長引数は気を付けて使うべし

  • 0個以上の引数を取る場合に、個数のチェック等が入って美しくない作りになるときがある。そういった場合は、取るべき引数を、以下のように2つに分解してやる。
package tryAny.effectiveJava;

public class Varargs {
    public static void main(String[] args) {
        System.out.println(min1(99));
        System.out.println(min2(1, 2, 3));

    }

    // ugly
    static int min1(int... args) {
        if (args.length == 0)
            throw new IllegalArgumentException("Too few arguments");
        int min = args[0];
        for (int i = 1; i < args.length; i++)
            if (args[i] < min)
                min = args[i];
        return min;
    }

    // better
    static int min2(int firstArg, int... remainingArgs) {
        int min = firstArg;
        for (int arg : remainingArgs) {
            if (arg < min) {
                min = arg;
            }
        }
        return min;
    }
}
  • 可変長引数を用いる場合には、配列のメモリ割り当てと初期化が必要となり、コストがかかる。もし、コストを減らす必要があり、引数の個数が3つ以下の場合が大多数であるとわかっている場合には、あらかじめ引数が0,1,2,3個の場合を作って対応するのがよい。
public void foo() { }
public void foo(int a1) { }
public void foo(int a1, int a2) { }
public void foo(int a1, int a2, int a3) { }
public void foo(int a1, int a2, int a3, int... rest) { }
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