import com.github.sbridges.contract.Contract; public class Calculator { private int result; public Calculator() { result = 0; } public int add(int a, int b) { Contract.requires(a > 0, "a must be positive"); Contract.requires(b > 0, "b must be positive"); int sum = a + b; Contract.ensures(sum > result, "sum must be greater than previous result"); result = sum; return sum; } }