Adding Null check is throwing a series of compile errors

Question

I was testing a program for CPU usage check and I got a null pointer exception, so I added null check. When I added null check I started getting series of errors. Here is the code:

double ideltime=Double.parseDouble(cpuIdle.trim());
**String idelTimeStr=formatter.format(ideltime);
if(idelTimeStr!=null)** 
double usuage=temp - Double.parseDouble(idelTimeStr);           
cpuUsage = formatter.format(usuage);

The Highlighted lines show the null check added. Compilation error after this null check is as follows:

CPUUsage.java:29: error: '.class' expected
                        double usuage=temp - Double.parseDouble(idelTimeStr);
                               ^
CPUUsage.java:29: error: not a statement
                        double usuage=temp - Double.parseDouble(idelTimeStr);
                        ^
CPUUsage.java:29: error: illegal start of expression
                        double usuage=temp - Double.parseDouble(idelTimeStr);
                                     ^
CPUUsage.java:29: error: ';' expected
                        double usuage=temp - Double.parseDouble(idelTimeStr);

Please help in resolving this.

Answer 1

You can ommit the curly braces when there is only one statement:

if (condition) 
    statement;

is identical to

if (condition) { 
    statement;
}

This is defined by the grammar in the JLS Chapter 14: Blocks and Statements. The relevant production terms are:

IfThenStatement:
    if ( Expression ) Statement
...

Statement:
    StatementWithoutTrailingSubstatement 
...

StatementWithoutTrailingSubstatement:
    Block 
    EmptyStatement 
    ExpressionStatement 
...

Finally, an ExpressionStatement is something like an assignment or a method invocation, but not a variable declaration. Variable declarations require a block.

JLS 14.2: Blocks:

A block is a sequence of statements, local class declarations, and local variable declaration statements within braces.

and JLS 14.4: Local Variable Declaration Statements:

Every local variable declaration statement is immediately contained by a block.

Since you are declaring double usuage = ... you need the curly braces in this case:

if(idelTimeStr != null) {
    double usuage=temp - Double.parseDouble(idelTimeStr);
}

is not identical to

if(idelTimeStr != null)
    double usuage=temp - Double.parseDouble(idelTimeStr);           

With the curly braces, your program is syntactically fine, but then you need to consider that the usuage variable is only visible within the block, so you will need to add more of your code inside the curly braces (or declare and initialize usuage with a default value outside of the if block).

In any case, I suggest to always use braces even if there is only one statement.