Constructors

Initialising an object's state when it is created: default constructor, parameterised and overloaded constructors, this(…) calls, attribute default values and copy constructors, in Java.

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Constructor: what is it?
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A block of code, named after the class and with no return type, run each time an object is created to initialise its state.
It is called with new and never called again on an existing object. Writing a return type, even void, would make it a plain method.
What are p.x and p.y after p is created?

```java
class Point {
int x;
int y;
Point(int x) {
this.x = x;
}
}

Point p = new Point(3);
`
- 3 and 0
- 3 and 3
- 0 and 0
- 3 and null
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3 and 0
The constructor only assigns this.x. y is an int: it keeps the default value 0, never null (reserved for references).
A constructor has the same name as the … and declares no …, not even void.
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A constructor has the same name as the class and declares no return type, not even void.
public void Point(int x) compiles, but it is an ordinary method that happens to be called Point: new Point(3) does not call it.
Default constructor: what is it?
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The parameterless constructor that the compiler adds by itself, only when the class declares none.
As soon as you write a constructor it disappears: new Point() no longer compiles if only Point(int x) exists.
Why write a constructor rather than assign the attributes one by one after the new?
- Attributes can only be assigned inside a constructor
- Without a constructor, the class cannot be instantiated
- The constructor runs faster than an assignment
- No object exists without first receiving a coherent starting state
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No object exists without first receiving a coherent starting state
The constructor guarantees the initialisation happens, once, before any other use. Without it, an attribute could be forgotten. A class with no constructor can still be instantiated thanks to the default constructor.

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