Assertion And Collection In Simplified Form:

By Author: James Peterson
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Assertions are statements in Java that enable you to test the assumptions made in a program during program execution. An assertion statement contains a Boolean expression that the programmer believes to be true at the time the statement is executed.

Assertions are used during testing of a program to ensure that the specified condition is true. The following syntax shows how to declare an assertion using assert statement in Java.

First Form of Assert:

assert expA;

Here expA should be a Boolean value.

Case 1:

When assertions are enabled
If the expA is true then program will run normally if it is false then an assertion error is thrown.

Case 2:

When assertions are disabled
Even if expA is false no error will be thrown. Simply, when assertions are disabled the assertion statement won’t work.

Example:

Consider the method needs a positive value as argument if not error need to be thrown.

void call(int a)
{
assert a>=0;
System.out.println(“a=”+a);
}

Say a= -1;
When assertions are enabled Assertion error ...
... is thrown. If assertions are disabled it prints a=-1;

Second form of assert:
assert expA : expB;

Here expB is error message. It will be printed when expA is true.

Example:

Consider the method needs a positive value as argument if not error need to be thrown.

void call(int a)
{
assert a>=0:”a value is less than zero”;
System.out.println(“a=”+a);
}

Say a= -1;
When assertions are enabled Assertion error is thrown with message “a value is less than zero”. If assertions are disabled it prints a=-1;
You can see reasonable no of questions on questions on Assertions in the certification exam. Practice through some good SCJP dumps and see the type of questions that appear in the actual exam.

Use of Collections:

Consider a scenario where there are two classes with class names One and Two. Let’s say In class Two there is only one method and three instance variables. In this situation when you call the method in class Two it can only return one instance variable value at a time. See the following example

Class One
{
Public static void main(String args[])
{
Two obj=new Two();
int a=obj.call(“a”);
int b=obj.call(“b”);
int c=obj.call(“c”);
System.out.println( a+“ ”+b+” ”+c);
}
}
Class Two
{
int x,y,z;
int call(String s)
if(s.equals(“a”))
return x;
if(s.equals(“b”))
return y;
if(s.equals(“c”))
return z;

}
}

In the above example for getting three instance variable values into class One from calss Two we have to invoke call() method of class Two thrice. If we use collections we can get all three variable values by invoking call() method of calss Two once. For that let’s modify the code by using Collections.

Class One
{
public static void main(String args[])
{
ArrayList obj2=new Two().call(); //calling call() method only once
int a=(Integer)obj2.get(0);
int b=(Integer)obj2.get(1);
int c=(Integer)obj2.get(2);
System.out.println(a+” ”b+” ”c);
}
}
Class Two
{
int x,y,z;
ArrayList call()
{
ArrayList obj=new ArrayList();
obj.add(x);
obj.add(y);
obj.add(z);
return obj; //returning three instance variables at once using //ArrayList object.
}
}

Here concept of autoboxing and unboxing has been used. One can practice through some good SCJP Mock Test and master the concept in a better way.

Author of the article is associated with Whizlabs, the global provider of Java and Project Management study material.