If you come across an expression that is hard to understand or it is duplicated in several places throughout your code, the Extract Variable refactoring Ctrl+Alt+V can help you deal with those problems placing the result of such expression or its part into a separate variable that is less complex and easier to understand. Plus, it reduces the code duplication.
If your Java version supports the pattern matching (Java 14 preview or higher) and you are extracting a cast expression under the instanceof check, IntelliJ IDEA may introduce a pattern variable at that instanceof check instead of a normal local variable. Also, you can inline pattern variable since the Java 14 version. In this case, all the occurrences will be replaced with old-style cast expression.
You can use the Introduce Variable refactoring to extract variadic arguments into a new slice variable.
In the editor, select an expression or its part that you want to extract. You can also position the caret within the expression, in this case IntelliJ IDEA offers you a list of potential code selections.
Press Ctrl+Alt+V or from the main menu, select Refactor | Extract/Introduce | Variable.
Select a name suggested in the popup or type your own and press Enter.
If IntelliJ IDEA finds more than one occurrence, it lets you specify a scope and extract just a part of the found occurrences and not just all of them.
You can also declare the variable you are extracting as final.
If you want to reassign the existing variable to a new one , press Ctrl+Alt+V. If you have more than one existing variable, IntelliJ IDEA displays a list to choose from.
You can press Shift+Tab to change a type of the variable.
If the In modal dialogs refactoring option is selected in the Refactorings area on the Code Editing page of the Settings/Preferences dialog Ctrl+Alt+S, IntelliJ IDEA opens the Extract Variable dialog for this refactoring.
Example
Let's extract the anotherClass.inValue() variable that occurs twice throughout the code and give it a name number.
Before
After
public void method() {
int a = 1;
...
int b = a + anotherClass.intValue();
int c = b + anotherClass.intValue();
}
public void method() {
int a = 1;
...
int number = anotherClass.intValue();
int b = a + number;
int c = b + number;
}
Before
After
func method() {
val a = 1
val b = a + anotherClass!!.intValue()
val c = b + anotherClass!!.intValue()
}
func method() {
val a = 1
val number = anotherClass!!.intValue()
val b = a + number
val c = b + number
}
Extract functional variable
This refactoring creates a functional expression for Java 1.8 and later versions, and an anonymous class for older versions of Java.
Select the code fragment, in this example, an argument of the println method.
In the main menu, select Refactor | Extract/Introduce | Functional Variable.
IntelliJ IDEA opens the Extract Functional Variable dialog.
Alternatively, from the context menu in the editor, select Refactor | Refactor ThisCtrl+Alt+Shift+T, and select Functional Variable.
When the selected code depends on instance fields, like in the example, the Pass field as params checkbox will appear and you can pass a parameter in a place of fields.
However, if for example, your selected code fragment depends on any local variable or a parameter
the corresponding entries would appear in the list.
When you deselect one of the parameters in the dialog, the corresponding local values will be used instead. Configure your options and click OK.
Choose an applicable functional interface from the popup.
If you want, change a name of the extracted variable if you don't want to use the name suggested in the list.
IntelliJ IDEA creates lambda that you can use further.
Example
Before
After
import java.util.List;
public class PrintAction implements Runnable {
private List<String> data;
public PrintAction(List<String> data) {
this.data = data;
}
public void run() {
System.out.println("Data: " + data.toString());
}
}
import java.util.List;
import java.util.function.Function;
public class PrintAction implements Runnable {
private List<String> data;
public PrintAction(List<String> data) {
this.data = data;
}
public void run() {
Function<List<String>, String> presenter = (p) -> "Data: " + p.toString();
System.out.println(presenter.apply(data));
}
}
