An object:
By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.
From a class, you can create several object (instance of a class) with different state but the same behaviours. A class provides the blueprint for objects; you create an object from a class. When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables.
A typical Java program creates many objects, which as you know, interact by invoking methods.
The phrase “instantiating a class” means the same thing as “creating an object.” When you create an object, you are creating an “instance” of a class, therefore “instantiating” a class.
Bundling code into individual software objects provides a number of benefits, including:
Every object is either:
datatype
For every type of object, the Java virtual machine instantiates an immutable instance of java.lang.Class.
The Java Machine has an internal id for each object created.
To verify that they are the same object use the == operator.
Debugger may also show an internal integer id such as 3128 or 3129 but this is an internal representation of their implementation. See ObjectReference uniqueID
Two objects may have different data but may be considered logically equals.
This happens with the help of a hash function.
There is two level of hash:
Object.hashCode()
System.identityHashCode(object)
Each of the following statements creates an object and assigns it to a variable:
Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);
Each of these statements has three parts (discussed in detail below):
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.
The phrase “instantiating a class” means the same thing as “creating an object.” When you create an object, you are creating an “instance” of a class, therefore “instantiating” a class.
The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:
int height = new Rectangle().height;
Interfaces and abstract classes can never be instantiated. You must always instantiate a concrete class but you can assign the resulting object to an interface.
Example where:
with this hierarchy
InterfaceFoo < AbstractFoo < ImplementationFoo
// Valid
InterfaceFoo foo= new ImplementationFoo ();
AbstractFoo foo = new ImplementationFoo ();
// Invalid
InterfaceFoo foo = new InterfaceFoo();
InterfaceFoo foo = new AbstractFoo();
AbstractFoo foo = new AbstractFoo();
For a class with a constructor without argument otherwise others reflection method must be used
Foo foo;
String fooClassName = getFooImplClass();
InterfaceFoo foo = (InterfaceFoo) Class.forName(fooClassName).newInstance();
Object o = new Integer("3");
if (o instanceof Integer)) {
System.out.println("This is an integer");
}
Some object-oriented languages require that you keep track of all the objects you create and that you explicitly destroy them when they are no longer needed. Managing memory explicitly is tedious and error-prone.
The Java platform allows you to create as many objects as you want (limited, of course, by what your system can handle), and you don't have to worry about destroying them. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is called garbage collection.
An object is drop:
Remember that a program can have multiple references to the same object; all references to an object must be dropped before the object is eligible for garbage collection.
See What does the equals method in Java?
Helper: https://guava.dev/releases/19.0/api/docs/com/google/common/base/Objects.ToStringHelper.html
@Override
public String toString()
{
return toStringHelper(this)
.add("tableName", tableName)
.add("type", type)
.add("columns", columns)
.toString();
}
this is a reference pointer to the object created by the class.
In an inner class, you can access the outer object with the qualified this - ie OuterClass.this from the InnerClass