This is because private variable can be accessed from all the methods contained by that class. Notice that the syntax here matches calling some object method. Unlike when calling a method, we refer to a field of an object, so the parentheses that indicate a method call are not written. Let's tweak the method olderThan of the Person class so that from here on out, we take use of the comparison functionality that date objects provide.
Now the concrete comparison of dates is implemented in the class that it logically based on the class names belongs to. While working with strings, we learned that strings must be compared using the equals method.
This is how it's done. With primitive variables such as int , comparing two variables can be done with two equality signs. This is because the value of a primitive variable is stored directly in the "variable's box". The value of reference variables, in contrast, is an address of the object that is referenced; so the "box" contains a reference to the memory location.
Using two equality signs compares the equality of the values stored in the "boxes of the variables" — with reference variables, such comparisons would examine the equality of the memory references. The method equals is similar to the method toString in the respect that it is available for use even if it has not been defined in the class. The default implementation of this method compares the equality of the references. Let's observe this with the help of the previously written SimpleDate class.
Variables first and first are equal Variables first and second are not equal Variables first and third are not equal Variables first and fourth are equal. There is a problem with the program above. Even though two dates first and second have exactly the same values for object variables, they are different from each other from the point of view of the default equals method.
If we want to be able to compare two objects of our own design with the equals method, that method must be defined in the class. The method equals is defined as a method that returns a boolean type value — the return value indicates whether the objects are equal. The equals method is implemented in such a way that it can be used to compare the current object with any other object.
The method receives an Object -type object as its single parameter — all objects are Object -type, in addition to their own type. The equals method first compares if the addresses are equal: if so, the objects are equal. After this, we examine if the types of the objects are the same: if not, the objects are not equal.
Next, the Object -type object passed as the parameter is converted to the type of the object that is being examined by using a type cast, so that the values of the object variables can be compared. Below the equality comparison has been implemented for the SimpleDate class.
Building a similar comparison functionality is possible for Person objects too. Below, the comparison has been implemented for Person objects that don't have a separate SimpleDate object.
Notice that the names of people are strings i. Let's examine how the equals method is used with lists. Let's assume we have the previously described class Bird without any equals method. Let's create a list and add a bird to it. After this we'll check if that bird is contained in it. We can notice in the example above that we can search a list for our own objects.
First, when the bird had not been added to the list, it is not found — and after adding it is found. When the program switches the red object into a new object, with exactly the same contents as before, it is no longer equal to the object on the list, and therefore cannot be found on the list.
The contains method of a list uses the equals method that is defined for the objects in its search for objects. In the example above, the Bird class has no definition for that method, so a bird with exactly the same contents — but a different reference — cannot be found on the list. Let's implement the equals method for the class Bird.
The method examines if the names of the objects are equal — if the names match, the birds are thought to be equal. We have seen methods return boolean values, numbers, and strings.
Easy to guess, a method can return an object of any type. In the next example we present a simple counter that has the method clone. The method can be used to create a clone of the counter; i. Immediately after the cloning operation, the values contained by the clone and the cloned object are the same.
Ok, probably 'A' is confusing you. How do you declare an int variable. Here, int is type of i, but 'i' is not referring to any value yet, so no value for 'i' bar default value. Think on same line. A reference variable can be of only one type and once declared that type can never be changed although the object it references can change A reference is a variable , so it can be reassigned to other object unless the reference is declared final A reference variable types determine the methods that can be invoked on the object the variable is referencing a reference variable can refer to any object of the same type as the declared reference or it can refer to subtype of the declared type a reference type can be declared class type or an interface type If the variable is declared as an interface type if the variable is declared of reference type it can reference any object of any class that implements the interface.
Putting some effort on your side will be best for you. I thought you are finding difficult to start up, so I helped you. All those answers you can get on web. Now if you get the error, the first step is to look for variables whose value could be null. Fortunately, the error message is useful: it tells which row caused the error. Just try it out yourself! Skip to content. Change Language. Related Articles. Table of Contents. Save Article. Improve Article. Like Article.
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Writing code in comment? An object contains elements such as variables and methods, as well as states and behaviors. A reference simply contains a sequence of bits. An object is never directly seen within the program, instead, the reference variable is assigned, and a reference to the object is created. An object is a real — world entity that holds some memory. A reference has no real meaning and its behavior is the same as that of a name of a person. It is the instance of a class and all the elements it contains are based on the blueprint of the class.
The memory reference is created alongside object. It is used in the format for object creation and is given a variable name. Objects have states and behaviors that can be changed, i. The reference variable value cannot be changed.
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