Comparison of Java to Kotlin for various simple methods.

_Java_vs_Kotlin.md

Comparison of Java 7 to Kotlin for various simple code snippets. Files with the same name do the same thing.

copy.java

public Person cloneWithNewName(Person original, String newName) {
  return new Person(
    original.id,
    newName,
    original.role,
    original.gender,
    original.seatingPosition,
    original.title
  )
}

copy.kt

fun cloneWithNewName(orignal: Person, newName: String): Person =
  original.copy(name = newName)

filter.java

/** Returns a subset of {@code list} containing only the items where {@code someBool == true} */
public final List<MyClass> filterList(List<MyClass> list) {
  List<MyClass> ret = new List<>(list.size());
  
  for(MyClass item : list) { 
    if(item.someBool) {
        ret.add(item);
      }
    }
    
  return ret;
}

filter.kt

/** Returns a subset of `list` containing only the items where `someBool == true` */
fun filterList(items:List<MyClass>) = items.filter { item -> item.someBool } // or { it.someBool }

KotlinComparisonClass.java

/** Example of a {@link Class} declaration */
public final class MyClass {
  private boolean someBool;
  
  public MyClass() {
  }
  
  public MyClass(boolean someBool) {
    this.someBool = someBool;
  }
  
  public boolean getSomeBool() {
    return someBool;
  }
  
  public void setSomeBool(@NonNull boolean value) {
    someBool = value;
  }
  
  @Override
  public String toString() {
    return "someBool:" + someBool;
  }
  
  // not shown: .copy override
  // not shown: .hashcode override
  // not shown: .equals override
}

KotlinComparisonClass.kt

/** Example of a [Class] declaration. */
data class MyClass(var someBool: Boolean = false)

nullability.java

/** Gets the second character in a string or, if not possible, returns some default value (which may be {@code null}) */
@Nullable
public final String getSecondCharOrDefault(@NonNull String str, @Nullable String defaultVal) {
  return str.length() > 2 ? str.get(1) : defaultVal;
}

nullability.kt

/** Gets the second character in a string or, if not possible, returns some default value (which may be `null`) */
fun getSecondCharOrDefault(str: String, defaultVal: String?) = if(str.length > 2) str[1] else defaultVal

rxJava.java

/** First prints "one dish", then "two dish" */
public final void printRxJavaStuff() {
  Observable.from(Arrays.asList("one fish", "two fish", "red fish", "blue fish"))
  .filter(new Func1<String, Boolean>() {
      @Override
      public Boolean call(String s) {
          return s.contains("one") || s.contains("two");
      }
  })
  .map(new Func1<String, String>() {
      @Override
      public String call(String s) {
          return s.replace("fish", "dish");
      }
      })
  .subscribe(new Action1<String>() {
      @Override
      public void call(String s) {
          Timber.v(s);
      }
  });
}

rxJava.kt

/** First prints "one dish", then "two dish" */
fun printRxJavaStuff() {
  Observable.from(asList("one fish", "two fish", "red fish", "blue fish"))
  .filter { it.contains("one") || it.contains("two") }
  .map { it.replace("fish", "dish") }
  .subscribe( { Timber.v(s) } )
}

yesOrNo.java

/** Returns {@code "yes"} if {@code value == true} or {@code "no"} if {@code value == false}. */
public boolean yesOrNo(boolean value) {
  return value ? "yes" : "no";
}

yesOrNo.kt

/** Returns `"yes"` if `value == true` or `"no"` if `value == false`. */
fun yesOrNo(value: Boolean) = if(value) "yes" else "no"