Question

In a class, you can create fields within them, as int bar in the following example.

Class foo{
    int bar;
    foo(int bar){
         bar = bar;
    }

    int getBar() {
         return bar;
    }   

    void setBar(int bar) {
         bar = bar;
    }
}

1. Every time I create a new foo object in another class, will that particular object (instance of foo) have a bar property that when changed with the setBar(), only impacts that instance and no other instances?

2. I often see people create getters and setters for properties like bar in the above example. If I feel lazy and I’m just writing code for myself/fun, can I just modify the bar property of any instance of this class by accessing the property FooInstance1.bar = 22; instead of having to write a setter and it will have the same effect as in question 1 (just the instance is changed)?

3. How does the Java compiler know what method is a constructor? By the fact that it has the same name as the class? Or by the fact that one does not specify a return value in the function header? Perhaps it doesn’t even matter what the compiler thinks is a constructor (no syntactic difference between regular function), it could be just a semantic thing that people use to differentiate the meaning of functions.

EDIT: I’m having a hard time selecting a best answer. I learned something new from everyone. Thanks!

Answer 1

1. Yes.
2. Yes, but it’s not encouraged. With OO, you’re trying to hide information about your instance variables, and provide them access on an as-needed basis. Also, setting the variable to public makes it susceptible to unintentional writes or unprivileged reads.
3. Java recognizes every method by its signature. A signature is composed of its name and it’s argument types. For example, the main method has a signature of main(String[]). Constructors are identified by them having the same case-sensitive name as the class.

As an aside, you may want to use this.bar = bar instead, or rename your incoming parameter entirely. Inside of the constructor, the scope of the bar variable is local to what was passed in, so your instance level bar hasn’t been assigned.