Java 天地·Core Java·Lecture Notes·Unitt 3 - Core Java Programming Concepts

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Content Page # 37

Declaring and assigning values to variables

Java is called a 'strongly typed' language because the Java compiler will not allow a variable to be referred to in a program unless it has been declared. In addition, values cannot be assigned to variables if the value and variable have different types. A variable declaration is a statement that defines the identifier (name) and type of a variable.

Examples of variable declarations include:

int numberOfRecords;
double temperature;

char firstInitial;
Note that each of these has a type (int, double, char) and an identifier (numberOfRecords, temperature, firstInitial).

It is also possible to give a variable an initial value (which can be overwritten with a different value elsewhere in the program). This is done by using the assignment operator = after the identifer, and provide the initial value:

int numberOfRecords = 20;
double temperature = 0;
char firstInitial = 'm';

As you might expect, the Java compiler will complain if you try to assign an initial value that is not compatible with the variables declared type. For example, compiling the program below results in an error message:

class TypeError
{
public static void main( String args[] )
{
int age = 0.05;
}
}

the output from the compiler was:

A:\Unit3\TypeError.java:15: possible loss of precision
found   : double
required: int
      int age = 0.05;
                ^
1 error

This error message occurs because, of course, a real value of 0.05 cannot be stored inside a variable of type int. Notice how the compiler has assumed that 0.05 is of type double automatically (rather than of type float).

What is interesting about this error message is that is tells us one way we could solve the type incompatibility problem! Being offered a solution, and in fact receiving an informative error message is, unfortunately, not always the case with Java. The solution offered is:

Explicit cast needed to convert double to int.

It is possible to covert a value from one type to another by a technique called casting.

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