Some Important Methods in JAVA
- 2. Accessors & Mutators ■ If we want a class/program external to our class to be able to access the value of our (always!) private member variables, we provide accessors. ■ If we want a class/program external to our class to be able to change the value of our (always!) private member variables, we provide mutators. ■ Accessors and mutators are both class methods. ■ An accessor (by convention) starts with get. A mutator (by convention) starts with set. ■ When we write a class, it is up to us to determine whether we provide access or allow mutations. ■ JAVA IDE provides functionality to generate accessors & mutators automatically. – Right click and select generate, then select getter and setter.
- 3. toString() Method ■ in Java, it is customary to create a string representation of an object in a class. ■ Let us create a string like the following example: – Name: John Jones – Age : 21 – Major Code:175 – Rank: Soph – Traditional Age
- 4. Test Class for Method ■ In line 4, we use the new operator to create an instance of (object in) the class Student. ■ The object's name is aStudent. It uses the constructor that accepts age, major code, name and rank. ■ Line 5 prints the toString value for aStudent. ■ Lines 6 and 7 use mutators to change values of two member variables ■ Line 8 prints the toString value for the updated object.
- 5. What we did… ■ We created a class called Student. ■ We gave Student – constructor(s) – Accessors – Mutators – toString() method
- 6. Output of Test Program Note the output of toString for the initial values of the object aStudent. Note the output after we used mutators to change age and name.
- 7. The equals() Method ■ We almost never use ==, !=, <=, >= when we compare Objects from any class. ■ We use the equals method which is also a method of Object. ■ In the class Object, the equals method returns true if the two Objects are located at the same memory location false otherwise ■ Fortunately, we can override the equals method and give our own definition of equality for objects in our class.
- 8. Equals Method for Person ■ Suppose we decide that two instances of Person are equal if they have the same name and same age. Note that we could have made a different definition of equality. ■ Below is the equals method for this definition. Note the signature!! ■ Other is the “other object” we are checking for equality with this.
- 9. Testing equals() Method Output: Equal
- 10. Better equals() Method ■ The equals method we wrote for the abstract class Shape is dangerous. ■ Why? The signature indicates that we will always be comparing one Shape to another for equality. ■ What if our user (inadvertently or unknowingly) used our equals method to compare a Shape to a Double or a Person or a Student or ... – Our program may Boom!
- 11. ■ The signature for equals should become: – public boolean equals(Object object) – Since everything is an Object, this is safe. ■ We use the instanceof operator to determine if object is an object in the Shape class. ■ If so, – Create a Shape object by typecasting object as a Shape. – Compare the just created Shape to this. ■ If not, – return false. Better equals() Method
- 12. Old way New way Example of equals() Method
- 13. Code Explanation ■ Line 18 is the amended prototype for the class Shape. ■ Line 20 uses instanceof to determine if the Object is a Shape. ■ If so, – Line 22 typecasts object as a Shape. – Line 23 calculates a boolean value to return. ■ Line 25 returns false in the case that object is not a Shape.
- 14. JAVA IDE Features ■ We learned about accessor & mutators, how to create toString(), and equals() methods. ■ The JAVA IDEs provide many features that will help you to write code easily & safely. ■ Research about your JAVA IDE and use it properly. ■ JAVA IDEs provide the functionality to generate accessor & mutators, toString(), and equals() method. ■ Just right click on code writing pane and select Generate, then you will see all the functions that provided by your JAVA IDE.
- 15. Thank You