java中的方法void

Question

public class PrimitiveVsReference {                    
    public static int k = 5;
    public static void main(String[] args) {       
        int[] a = {7,8}; 
        String s = "Hello";
        update(k,a,s);
        System.out.println(k + "\n" + a[0] + "\n" + a[1] + "\n" + s);
    }
    public static void update(int k, int[] a, String s){
        a[k-4]++;
        k++;
        s = "World";
    }
}

此方法返回5 7 9 hello

为什么不返回6 7 9 World ？