Arrays#binarySearch で値が見つからなかった場合のマイナス値をビット反転することで、挿入位置が分かる。

ArraysBinarySearchTest.java

import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.MatcherAssert.assertThat;

import java.util.Arrays;

import org.junit.Test;

/**
 * ArraysBinarySearchTest
 */
public class ArraysBinarySearchTest {

    @Test
    public void binarySearch() {
        double[] da = new double[] { 1.0, 2.0, 3.0 };
        int a1 = Arrays.binarySearch(da, 0.5);
        int a2 = Arrays.binarySearch(da, 1.0);
        int a3 = Arrays.binarySearch(da, 1.5);
        int a4 = Arrays.binarySearch(da, 2.0);
        int a5 = Arrays.binarySearch(da, 2.5);
        int a6 = Arrays.binarySearch(da, 3.0);
        int a7 = Arrays.binarySearch(da, 3.5);

        assertThat(a1, is(-1));
        assertThat(a2, is(0));
        assertThat(a3, is(-2));
        assertThat(a4, is(1));
        assertThat(a5, is(-3));
        assertThat(a6, is(2));
        assertThat(a7, is(-4));

        assertThat(~a1, is(0));
        assertThat(~a3, is(1));
        assertThat(~a5, is(2));
        assertThat(~a7, is(3));
    }

}