Kurage Notes

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References ​

Primitive Types ​

  • byte
  • short
  • int
  • long
  • float
  • double
  • boolean
  • char

Declaring Variables ​

When you declare a variable of a certain type, Java finds a contiguous block with exactly enough bits to hold a thing of that type. For example, if you declare an int, you get a block of 32 bits. If you declare a byte, you get a block of 8 bits. Each data type in Java holds a different number of bits.

Java does not write anything into the reserved box when a variable is declared. In other words, there are no default values. As a result, the Java compiler prevents you from using a variable until after the box has been filled with bits using the = operator.

The Golden Rule of Equals ​

When you write y = x, you are telling the Java interpreter to copy the bits from x into y. This simple idea of copying the bits is true for ANY assignment using = in Java.

Reference Types ​

Every type other than a primitive type is a reference type. Arrays of primitive type are also references.

Object Instantiation ​

When we instantiate an Object using new, Java first allocates a box for each instance variable of the class, and fills them with a default value.

The Object we've created is anonymous, in the sense that it has been created, but it is not stored in any variable. Let's now turn to variables that store objects.

Reference Variable Declaration ​

When we declare a variable of any reference type, Java allocates a box of 64 bits, no matter what type of object. This box contains the address of the object of that type.

To assign the reference variable:

  • Calling a new operator returns the address of the newly created object.
  • Assigning by another reference of the same type.
  • We can also assign the special value null to a reference variable, corresponding to all zeros.
You should think of Reference Types as Pointers in C++

Parameter Passing ​

When you pass parameters to a function, you are also simply copying the bits. Copying the bits is usually called "pass by value". In Java, we always pass by value.

  • Pass the value of Reference (Point to the same address)