Javapoet: Define Anonymous inner classes

'TypeSpec.anonymousClassBuilder("")' is used to define anonymous inner class.

TypeSpec comparatorType = TypeSpec.anonymousClassBuilder("")
    .addSuperinterface(ParameterizedTypeName.get(Comparator.class, String.class))
    .addMethod(MethodSpec.methodBuilder("compare").addAnnotation(Override.class).addModifiers(Modifier.PUBLIC)
        .addParameter(String.class, "a").addParameter(String.class, "b").returns(int.class)
        .addStatement("return $N.length() - $N.length()", "a", "b").build())
    .build();

MethodSpec sortByLengthMethodSpec = MethodSpec.methodBuilder("sortByLength").addModifiers(Modifier.STATIC)
.addParameter(ParameterizedTypeName.get(List.class, String.class), "inputList")
.addStatement("$T.sort($N, $L)", Collections.class, "inputList", comparatorType).build();

Above snippet generates following method definition.

static void sortByLength(List<String> inputList) {
    Collections.sort(inputList, new Comparator<String>() {
      @Override
      public int compare(String a, String b) {
        return a.length() - b.length();
      }
    });
  }

Find the below working application.

 

AnonymousClassDemo.java

package com.sample.app;

import java.io.IOException;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import javax.lang.model.element.Modifier;

import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.ParameterizedTypeName;
import com.squareup.javapoet.TypeSpec;

public class AnonymousClassDemo {

  public static void main(String args[]) throws IOException {
    TypeSpec comparatorType = TypeSpec.anonymousClassBuilder("")
        .addSuperinterface(ParameterizedTypeName.get(Comparator.class, String.class))
        .addMethod(MethodSpec.methodBuilder("compare").addAnnotation(Override.class)
            .addModifiers(Modifier.PUBLIC).addParameter(String.class, "a").addParameter(String.class, "b")
            .returns(int.class).addStatement("return $N.length() - $N.length()", "a", "b").build())
        .build();

    MethodSpec sortByLengthMethodSpec = MethodSpec.methodBuilder("sortByLength").addModifiers(Modifier.STATIC)
        .addParameter(ParameterizedTypeName.get(List.class, String.class), "inputList")
        .addStatement("$T.sort($N, $L)", Collections.class, "inputList", comparatorType).build();

    MethodSpec mainMethodSpec = MethodSpec.methodBuilder("main").addModifiers(Modifier.PUBLIC, Modifier.STATIC)
        .returns(void.class).addParameter(String[].class, "args")
        .addStatement("sortByLength(asList(\"a\", \"abcd\", \"ab\"))").build();

    TypeSpec helloWorldTypeSpec = TypeSpec.classBuilder("Test").addModifiers(Modifier.PUBLIC, Modifier.FINAL)
        .addMethod(mainMethodSpec).addMethod(sortByLengthMethodSpec).build();

    String packageName = "com.sample.app";
    JavaFile javaFile = JavaFile.builder(packageName, helloWorldTypeSpec).addStaticImport(Arrays.class, "asList")
        .build();

    javaFile.writeTo(System.out);
  }

}

Generated Code

package com.sample.app;

import static java.util.Arrays.asList;

import java.lang.Override;
import java.lang.String;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public final class Test {
  public static void main(String[] args) {
    sortByLength(asList("a", "abcd", "ab"));
  }

  static void sortByLength(List<String> inputList) {
    Collections.sort(inputList, new Comparator<String>() {
      @Override
      public int compare(String a, String b) {
        return a.length() - b.length();
      }
    });
  }
}

 

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