Extract method

The Extract Method refactoring lets you take a code fragment that can be grouped, move it into a separated method, and replace the old code with a call to the method.

When you extract the method you need to check for variables. If there is one output variable, it is used as a return value for the extracted method. In case there are multiple output variables, the Extract Method refactoring may not be applied, and the error message appears.

There are several workarounds to allow Extract Method work in this case. For example, you may introduce a special data-class that contains all output values.

Extract method

Select a code fragment you want to extract to a method.
Press Ctrl+Alt+M or go to Refactor | Extract | Method in the main menu.
Alternatively, on the toolbar that appears, click Extract and select Method.
In the dialog that opens, configure method options, such as visibility, parameters, and so on. You can also change the name of the method if you need. Click OK.
If IntelliJ IDEA detects a duplicated code fragment that can be replaced with a call to extracted method, it will offer you to preview your code, compare it side by side and make changes if needed. You can also right-click the duplicate code fragment and select Exclude to remove such code from refactoring.
When you are ready, click Do Refactor to complete your refactoring.
Note that if it is just one duplicate, IntelliJ IDEA will replace it silently.
Finally, if IntelliJ IDEA detects code that is only partially duplicated, it suggests extracting a parameter to proceed with the refactoring.

Example

Let's extract the a+b expression into a method (function for Kotlin), and replace duplicates .

Before	After
public void method() { int a=1; int b=2; int c=a+b; int d=a+c; }	public void method() { int a=1; int b=2; int c=add(a,b); int d=add(a,c); } ... private int add(int a, int b) { return a+b; }

Before	After
fun method(){ val a = 1 val b = 2 val c = a + b val d = a + b }	fun method(){ val a = 1 val b = 2 val c = add(a, b) val d = add(a, b) } private fun add(a: Int, b: Int) = a + b

Extract a method using Java records

Starting with the Java 16 version, you can extract a method using Java records. That might be helpful when you have multiple variables. In these cases, the IDE first suggests wrapping these variables into a new record or bean class and then performing method extraction.

                    class SomeClass {

                        public static void main(String[] args) {
                            int values[] = {1, 2, 3, 4, 5};
                            int sum = 0;
                            int num = 0;

                            for (int n : values) {
                                sum += n;
                                num++;
                            }

                            System.out.println("Sum: " + sum + " - Num: " + num);
                        }
                    }
                

                    class SomeClass {

                        public static void main(String[] args) {
                            Statistics result = getStatistics();

                            System.out.println("Sum: " + result.sum() + " - Num: " + result.num());
                        }

                        private static Statistics getStatistics() {
                            int values[] = {1, 2, 3, 4, 5};
                            int sum = 0;
                            int num = 0;

                            for (int n : values) {
                                sum += n;
                                num++;
                            }
                            Statistics result = new Statistics(sum, num);
                            return result;
                        }

                        private record Statistics(int sum, int num) {
                        }
                    }
                

Last modified: 11 February 2024