Using static variables in local scope in Java.

Test.java

import java.util.regex.Pattern;
 
public class Test {
    public static void main(String[] args) {
        check("a");
        check("b");
    }
     
    static Pattern compile(String pattern) {
        System.out.println("compile(" + pattern + ")");
        return Pattern.compile(pattern);
    }
     
    static void check(String string) {
 
        // Compiling the pattern only once: Good
        // Keeping the pattern local to the method: Good
        // Capturing scope: Egh...
        var patterns = new Object() { 
            static final Pattern P_CHECK = compile("a");
        };
         
        System.out.println("check(" + string + "): "
            + patterns.P_CHECK.matcher(string).find());
    }
}

Test2.java

import java.util.regex.Pattern;
 
public class Test {
    public static void main(String[] args) {
        check("a");
        check("b");
    }
     
    static Pattern compile(String pattern) {
        System.out.println("compile(" + pattern + ")");
        return Pattern.compile(pattern);
    }
     
    static void check(String string) {
 
        // Compiling the pattern only once: Good
        // Keeping the pattern local to the method: Good
        // Does not capture the scope: Good
        class Patterns {
            static final Pattern P_CHECK = compile("a");
        }
        
        System.out.println("check(" + string + "): "
            + Patterns.P_CHECK.matcher(string).find());
    }
}

Bytcode of the first version:

▶ javap -c Test\$1 
Compiled from "Test.java"
class Test$1 {
  static final java.util.regex.Pattern P_CHECK;

  Test$1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  static {};
    Code:
       0: ldc           #7                  // String a
       2: invokestatic  #9                  // Method Test.compile:(Ljava/lang/String;)Ljava/util/regex/Pattern;
       5: putstatic     #15                 // Field P_CHECK:Ljava/util/regex/Pattern;
       8: return
}

▶ javap -c Test
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #7                  // String a
       2: invokestatic  #9                  // Method check:(Ljava/lang/String;)V
       5: ldc           #15                 // String b
       7: invokestatic  #9                  // Method check:(Ljava/lang/String;)V
      10: return

  static java.util.regex.Pattern compile(java.lang.String);
    Code:
       0: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
       3: aload_0
       4: invokedynamic #23,  0             // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
       9: invokevirtual #27                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      12: aload_0
      13: invokestatic  #32                 // Method java/util/regex/Pattern.compile:(Ljava/lang/String;)Ljava/util/regex/Pattern;
      16: areturn

  static void check(java.lang.String);
    Code:
       0: new           #38                 // class Test$1
       3: dup
       4: invokespecial #40                 // Method Test$1."<init>":()V
       7: astore_1
       8: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
      11: aload_0
      12: aload_1
      13: pop
      14: getstatic     #41                 // Field Test$1.P_CHECK:Ljava/util/regex/Pattern;
      17: aload_0
      18: invokevirtual #45                 // Method java/util/regex/Pattern.matcher:(Ljava/lang/CharSequence;)Ljava/util/regex/Matcher;
      21: invokevirtual #49                 // Method java/util/regex/Matcher.find:()Z
      24: invokedynamic #55,  0             // InvokeDynamic #1:makeConcatWithConstants:(Ljava/lang/String;Z)Ljava/lang/String;
      29: invokevirtual #27                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      32: return
}

The generated class Test$1 is identical to the Test$Patterns class, but the check implementation is unnecessarily creating a new instance of Test$1, resulting in 8 extra, unnecessary bytecode instructions.