assertion

assertion

Introduction

• State something in a strong, confident, and forceful way,
• When you assert something, you believe that something to be true,
• But it does not make that something always true,
• Sometimes you may be wrong and that something may be false, even if you assert it to be true,

Why assertion

• Helps to debug code in development and testing phase,
• Used for detecting logical errors in a program,

assertion

• Can be written in 2 ways:
  • assert booleanAssertionExpression;
    • This form is enough in almost all cases,
  • assert booleanAssertionExpression : errorMessageExpression;
    • errorMessageExpression: allows writing custom error message. Can be any data type,
• If booleanAssertionExpression evaluates to true, no action is taken. Code is executed normally,
• But if evaluates to false, java.lang.AsssertionError is thrown,
• Assertion is by default disabled. You must run your code using below to see assertion in action:

  java -ea YourClassName

• Ex:

  java -ea assertion\Test.java

• Ex(See Test.java):

  private static void assertType1(){
      int a = getNumber(2);
  
      assert a==4;
      /*
          when we write 4, we know how getNumber(int) works. It helps to debug code.
          Because if a != 4, then there must have error in getNumber(int) function
      */
  
      // assert a == 6; // will throw runtime exception
      System.out.println(a);
  }
  
  private static void assertType2(){
      int a = getNumber(2);
  
      assert a==6 : "Something is wrong in getNumber(int)";
  
      System.out.println(a); // java.lang.AssertionError: Something is wrong in getNumber(int)
  }

• Must have to run using above command. Otherwise, assertion will be ignored,

Another example:

• method definition

    private static int calculateAge(LocalDate localDate){
        ...
        return (int)age;
    }

• Calling like:

  LocalDate dob = LocalDate.of(2001,3,21);
  int age = calculateAge(dob);
  
  assert age >= 22;
  System.out.println("I was right"); // executed