java计算100以内的正整数和的三种方法

    /**
     * <p>暴力解法</p>
     * @param startInclusive
     * @param endExclusive
     * @return
     */
    public int sumByDirect(int startInclusive, int endExclusive){
        int sum = 0;
        for (int i = startInclusive; i < endExclusive; i++) {
            sum += i;
        }
        return sum;
    }

    /**
     * <p>流式编程</p>
     * @param startInclusive
     * @param endExclusive
     * @return
     */
    public int sumByStream(int startInclusive, int endExclusive){
        return IntStream.range(startInclusive, endExclusive).sum();
    }

    /**
     * <p>利用求和公式</p>
     * @param startInclusive
     * @param endExclusive
     * @return
     */
    public int sumByFormula(int startInclusive, int endExclusive){
        return ((startInclusive + endExclusive - 1) * (endExclusive - startInclusive) )  >> 1;
    }

    @Test
    public void Test() {
        System.out.println("sumByDirect=" + sumByDirect(1, 101));
        System.out.println("sumByStream=" + sumByStream(1, 101));
        System.out.println("sumByFormula=" + sumByFormula(1, 101));
    }

sumByDirect=5050
sumByStream=5050
sumByFormula=5050