为什么 three = one + two；不能int three = one + two 吗

        int three = one + two;

        System.out.println("three = one + two ==>" + three);

        int three += one;

        System.out.println("three += one ==>" + three);

        int three -= one;

        System.out.println("three -= one ==>" + three);

        int three *= one;

        System.out.println("three *= one ==>" + three);

        int three /= one;

        System.out.println("three /= one ==>" + three);

        int three %= one;

        System.out.println("three %= one==>" + three);

为什么不要加int。。。。。