Java test if variable defined

Java if statements

The Java if statement enables your Java programs to make decisions about what code to execute depending on the state of variables, or values returned from methods. Here is a simple Java if example:

boolean isValid = true; if ( isValid ) < System.out.println("it is valid"); >else

The if statement in this example tests the boolean variable isValid and based on its value (either true or false ) it executes one of two different blocks of code. If the isValid variable has the value of true , the first block is executed. If not, the code inside the else block is executed.

The expression inside the parentheses is called the condition. The condition can be any Java expression, as long as the result of the expression is a boolean result (either true or false ).

In the example above, the condition was whether the isValid variable was true or false.

If the block of code to be executed is just a single statement, you do not need the brackets < >around them, in the if statements. Here is an example:

if ( isValid ) System.out.println("it is valid"); else System.out.println("it is not valid");

However, it is good practice to put the brackets around the statements, even if there is only one statement to execute. Often during development you may start with a single statement that needs to be executed inside an if or else block, but later have to add more statements to the blocks. This can lead to errors that are hard to spot. Look at this if statement:

if( isValid) System.out.println("it is valid");

Now imagine I have to increment a valid counter if isValid is true . Naively I might change the code to this:

if( isValid) validCount++; System.out.println("it is valid");

But now only the validCount++ statement belongs to the if statement. The System.out.println() statement will always be executed. Or, imagine if I had switched the statements like this:

if( isValid) System.out.println("it is valid"); validCount++;

Now only the System.out.println() statement belongs to the if statement. The validCount++ statement will always be executed.

To avoid this error I almost always put the brackets around the blocks to execute, even if there is only one statement to execute in the block. Here is how that could look:

When the brackets are there, it is easier to remember to insert new statements inside the brackets.

Conditional Operators

Java has a set of conditional operators you can use, which result in a value of either true or false . These are:

The == operator tests if two values are equal to each other. For instance:

long var1 = 2; long var2 = 5; if(var1 == var2) < //. >

If the two variables, var1 and var2 , are equal, the expression var1 == var2 is evaluated to true . Otherwise the expression is evaluated to false .

The != operator does the exact opposite of the == operator. If the two variables are not equal, the expression is evaluated to true . If the two variables are equal, the expression is evaluated to false .

The >= operator works like the > operator except it also evaluates to true if the two variables are equal to each other.

Comparing Variables and Constants

In the examples earlier in this text I have only shown comparisons of either constants to constants, or variables to variables. But, you can also compare constants to variables. Here are two examples:

int var1 = 50; if(var1 > 10) < //. >if(99 

Methods as Conditions

You can also use the return value of a method as condition in an if statement. Here is how:

public void methodOne (String input) < if ( isValid(input) ) < System.out.println(input + " is valid"); >else < System.out.println(input + " is not valid"); >> public boolean isValid(String value) < if( value.equals("123") ) < return true; >return false; >

This example actually contains two if statements with methods as conditions. First the

which tests the output of the isValid(input) method, for a true or false result.

Second, inside the isValid() method the String.equals() method is used to test for equality to a certain string value. This is the if statement that tests it:

The isValid() method could actually have been written in a shorter way. Here is how:

public boolean isValid(String value)

Now the isValid() method returns the value returned by the value.equals() method call.

You could also switch the string «123» and value variable in the statement, like this:

public boolean isValid(String value)

This version actually has the advantage, that if value is null (does not point to a String object, but to nothing), this version will not result in a NullPointerException .

Chaining if Statements

It is possible to chain if statements, to create a decision tree. Here is an example:

if( name.equals("john")) < //. >else if ( name.equals("jane")) < //. >else if ( name.equals("Linda")) < //. >else < //. >

In the example above, else if statements are chained, one after another. Actually, this chained if statement is just an if statement executed in an else block, without the brackets < >, as I showed you that you can, in the beginning of this text. The above code is actually equivalent to this:

if( name.equals("john")) < //. >else < if ( name.equals("jane")) < //. >else < if ( name.equals("Linda")) < //. >else < //. >> >

As you can see, the first version is actually easier to read. This is the one exception I normally have to the rule of always embedding the statements of the if and else inside brackets. In this case I prefer the first version. It is easier to read, write, and does not often result in programming errors.

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How to check if a variable is defined in Java

In this post, we will be discussing a variable, how it is declared and how to check its presence in Java. Every program made out of a language contains data that is executed and implemented in the way it is coded. Without data, there will be nothing for the computer to understand and run the login on. Let us first understand what a variable is:

Variable in Java

A variable in simple words is a vessel that holds something which is used in the logic of an executable program. Java is a language that cares about the type of data. Variables contain data in them and these variables should have a type and a name given to the type.
Example: int ant; – here ‘ int ‘ is the type of data and ‘ant’ is the name given to the declared integer.

Now let’s see how to declare and check a variable:
Variables can be declared in three ways:

1. As object state (instance variables)
2. Within the method (local variables)
3. As return types

If a variable is not properly declared, that is- if the type of the variable or the format of the name given to the variable is incorrect then the program will not give the expected output and it won’t be executed as it has problems interpreting the data input.

Check if a variable exists or not in a program in Java

To check if a variable is declared in Java or not, in a simple way an if conditional can be used in the following way:

if (variablename != NULL) else 

In the above code, in the if conditional if the declared variable is not null then it will print “Variable declared” and if it is not declared the screen output will print “Variable not declared”. This is a simple way to check if a variable is declared in a Java program.

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Java check if variable is set code example

Solution 2: Instance variables or fields, along with static variables, are assigned default values based on the variable type: int: char: or double: boolean: reference: Just want to clarify that local variables (ie. declared in block, eg. method, for loop, while loop, try-catch, etc.) are not initialized to default values and must be explicitly initialized. Indeed, you may even want to change the above code to: The difference is also visible for local variables, which can’t be read before they’ve been «definitely assigned» — but one of the values which they can be definitely assigned is null (for reference type variables):

Is there a way to check if a variable is defined in Java?

The code won’t compile if you try to use an undefined variable, because, In Java, variables must be defined before they are used.

But note that variables can be null, and it is possible to check if one is null to avoid NullPointerException :

if (variableName != null) < //Do something if the variable is declared. >else < //Do something if the variable doesn't have a value >

I think that should do it.

It will throw an exception if we try to use an undefined variable in java. To over come these make use of wrapper Class and assigned it to null.

Integer a = null; //correct int a = null;//error 

Java check to see if a variable has been initialized, Assuming you’re interested in whether the variable has been explicitly assigned a value or not, the answer is «not really». There’s absolutely no difference between a field (instance variable or class variable) which hasn’t been explicitly assigned at all yet, and one which has been assigned its default value — …

Java check to see if a variable has been initialized

Assuming you’re interested in whether the variable has been explicitly assigned a value or not, the answer is «not really». There’s absolutely no difference between a field (instance variable or class variable) which hasn’t been explicitly assigned at all yet, and one which has been assigned its default value — 0, false, null etc.

Now if you know that once assigned, the value will never reassigned a value of null, you can use:

(and that also avoids a possible NullPointerException ) but you need to be aware that «a field with a value of null» isn’t the same as «a field which hasn’t been explicitly assigned a value». Null is a perfectly valid variable value (for non-primitive variables, of course). Indeed, you may even want to change the above code to:

The difference is also visible for local variables, which can’t be read before they’ve been «definitely assigned» — but one of the values which they can be definitely assigned is null (for reference type variables):

// Won't compile String x; System.out.println(x); // Will compile, prints null String y = null; System.out.println(y); 

Instance variables or fields, along with static variables, are assigned default values based on the variable type:

• int: 0
• char: \u0000 or 0
• double: 0.0
• boolean: false
• reference: null

Just want to clarify that local variables (ie. declared in block, eg. method, for loop, while loop, try-catch, etc.) are not initialized to default values and must be explicitly initialized.

Java — Variable Check if Contains, I want to search if a variable contains a certain word or words and then if it does set a new variable. For example if someone signs up for email and they have Gmail, I want to set a new variable for use later. I’m self taught and obviously new to java and would appreciate any feedback. Thanks.

In Activiti, how do I check if a variable is set?

Use the following expression:

You have to set startTime variable in both cases;

variables.put("startTime", startTime); ProcessInstance instance = runtimeService.startProcessInstanceByKey(processKey, variables); 

variables.put("startTime", null); ProcessInstance instance = runtimeService.startProcessInstanceByKey(processKey, variables); 

Then check variable in gateway

You can use http://www.activiti.org/userguide/#bpmnTimerStartEvent

Java — Check Not Null/Empty else assign default value, I am trying to simplify the following code. The basic steps that the code should carry out are as follows: Assign String a default value ; Run a method ; If the method returns a null/empty string leave the String as default ; If the method returns a valid string set the String to this result; A Simple example would be:

How to detect if a variable has changed?

Since you want to find and perform some action only if the value changes, I would go with setXXX, for example:

You can use getter/setter with dirty bit associated with each field. mark it dirty if the value is changed through setter, and force user to use setters

another way is use AOP to intercept changing of fields, AspectJ for example, you could have a look a http://www.eclipse.org/aspectj/doc/released/progguide/semantics-pointcuts.html

Java check if variable is not null Code Example, Java answers related to “java check if variable is not null” check if optional is empty java; java check if int is null; how to check null and empty string in java; java notnull returns null; how to assign null to a variable in Java

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