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How Kotlin implements
features Java doesn’t have
@lvla0805
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Ѫᖒ๖ (Moyuru Aizawa)
lvla lvla0805
- Kotlin engineer at CyberAgent, Inc.
- FRESH!
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Data Classes
Sealed Classes
Extension
Functions
Inline Functions
Default Arguments
Named Arguments
Delegation
Properties
Auto Casts
Type Alias
Covariance
Null Safety
Elvis Operator
Spread Operator
Operator
Overloading
Type Inference
DSL Support
Type-Safe Builders
Coroutines
String Interpolation
Rages
Map Array Syntax
Destructuring
KDoc
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No content
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Bytecode
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NonNull / Nullable
Extension Function
Named Argument / Default Argument
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NonNull / Nullable
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fun nonNullInt(number: Int) {
number.toString()
}
NonNull: Kotlin
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public final nonNullInt(I)V
L0
LINENUMBER 7 L0
ILOAD 1
INVOKESTATIC java/lang/String.valueOf (I)Ljava/lang/String;
POP
…
NonNull: Bytecode
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public final nonNullInt(I)V
L0
LINENUMBER 7 L0
ILOAD 1
INVOKESTATIC java/lang/String.valueOf (I)Ljava/lang/String;
POP
…
NonNull: Bytecode
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public final void nonNullInt(int number) {
String.valueOf(number);
}
NonNull: Java
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fun nullableInt(number: Int?) {
number?.toString()
}
Nullable: Kotlin
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public final nullableInt(Ljava/lang/Integer;)V
@Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0
L0
LINENUMBER 7 L0
ALOAD 1
DUP
IFNULL L1
INVOKEVIRTUAL java/lang/Integer.intValue ()I
INVOKESTATIC java/lang/String.valueOf (I)Ljava/lang/String;
POP
GOTO L2
L1
POP
…
Nullable: Bytecode
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public final nullableInt(Ljava/lang/Integer;)V
@Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0
L0
LINENUMBER 7 L0
ALOAD 1
DUP
IFNULL L1
INVOKEVIRTUAL java/lang/Integer.intValue ()I
INVOKESTATIC java/lang/String.valueOf (I)Ljava/lang/String;
POP
GOTO L2
L1
POP
…
Nullable: Bytecode
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public final nullableInt(Ljava/lang/Integer;)V
@Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0
L0
LINENUMBER 7 L0
ALOAD 1
DUP
IFNULL L1
INVOKEVIRTUAL java/lang/Integer.intValue ()I
INVOKESTATIC java/lang/String.valueOf (I)Ljava/lang/String;
POP
GOTO L2
L1
POP
…
Nullable: Bytecode
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public final void nullableInt(@Nullable Integer number) {
if(number != null) {
String.valueOf(number.intValue());
}
}
Nullable: Java
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fun nonNullString(string: String) {
string.length
}
NonNull: Kotlin
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public final nonNullString(Ljava/lang/String;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
ALOAD 1
LDC "string"
INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull
(Ljava/lang/Object;Ljava/lang/String;)V
L1
LINENUMBER 7 L1
ALOAD 1
INVOKEVIRTUAL java/lang/String.length ()I
POP
…
NonNull: Bytecode
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public final nonNullString(Ljava/lang/String;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
ALOAD 1
LDC "string"
INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull
(Ljava/lang/Object;Ljava/lang/String;)V
L1
LINENUMBER 7 L1
ALOAD 1
INVOKEVIRTUAL java/lang/String.length ()I
POP
…
NonNull
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public final nonNullString(Ljava/lang/String;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
ALOAD 1
LDC "string"
INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull
(Ljava/lang/Object;Ljava/lang/String;)V
L1
LINENUMBER 7 L1
ALOAD 1
INVOKEVIRTUAL java/lang/String.length ()I
POP
…
NonNull: Bytecode
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public final void nonNullString(@NotNull String string) {
Intrinsics.checkParameterIsNotNull(string, "string");
string.length();
}
NonNull: Java
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private fun nonNullString(string: String) {
string.length
}
NonNull: Kotlin
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private final nonNullString(Ljava/lang/String;)V
L0
LINENUMBER 7 L0
ALOAD 1
INVOKEVIRTUAL java/lang/String.length ()I
POP
NonNull: Bytecode
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private fun nullableString(string: String?) {
string?.length
}
Nullable: Kotlin
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private final static nullableString(Ljava/lang/String;)V
L0
LINENUMBER 2 L0
ALOAD 0
DUP
IFNULL L1
INVOKEVIRTUAL java/lang/String.length ()I
POP
GOTO L2
L1
POP
L2
L3
LINENUMBER 3 L3
RETURN
…
Nullable: Bytecode
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private final void nullableString(String string) {
if(string != null) {
string.length();
}
}
Nullable: Java
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Conclusion of NonNull/Nullable
Primitive NonNull -> Primitive
Primitive Nullable -> BoxType
ClassType NonNull -> ClassType, Check Null
ClassType Nullable -> ClassType
Safe Call -> IFNULL
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Extension Function
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fun String.println() {
println(this)
}
"Hello World".println() // out: Hello World
Extension Function
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Package Level
Extension Function
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// in StringExtension.kt
fun String.println() {
println(this)
}
Package Level Extension Function: Kotlin
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public final class StringExtensionKt {
public final static println(Ljava/lang/String;)V
…
}
Package Level Extension Function: Bytecode
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public final class StringExtensionKt {
public final static println(Ljava/lang/String;)V
…
}
Package Level Extension Function: Bytecode
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public final class StringExtensionKt {
public final static println(Ljava/lang/String;)V
…
}
Package Level Extension Function: Bytecode
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public final class StringExtensionKt {
public static final void println(@NotNull String $receiver) {
Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
System.out.println($receiver);
}
}
Package Level Extension Function: Java
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Class Level Extension Function
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class Foo {
fun String.println() {
println(this)
}
}
Class Level Extension Function: Kotlin
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public final class Foo {
public final println(Ljava/lang/String;)V
…
}
Class Level Extension Function: Bytecode
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Extension Function
Package Level Extension Function -> Static Method
Class Level Extension Function -> Method
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Package Level Extension Function
Package Level Extension Function -> Static Method
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Package Level Extension Function
Public Package Level Extension Function -> Public Static Method
Private Package Level Extension Function -> Private Static Method
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Scope of Package Level Extension
Public Package Level Extension Function -> Anywhere
Private Package Level Extension Function -> Same file
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Private Package Level Extension Function
Private Package Level Extension Function -> Private Static Method
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// in Bar.kt
private fun Foo.doSomething() {
}
class Foo {
init {
doSomething()
}
}
Private Package Level Extension Function: Kotlin
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public final class FooKt {
private final static doSomething(LFoo;)V
…
public final static synthetic access$doSomething(LFoo;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
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ALOAD 0
INVOKESTATIC FooKt.doSomething (LFoo;)V
RETURN
L1
LOCALVARIABLE $receiver LFoo; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 1
Private Package Level Extension Function: Bytecode
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public final class FooKt {
private final static doSomething(LFoo;)V
…
public final static synthetic access$doSomething(LFoo;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
LINENUMBER 1 L0
ALOAD 0
INVOKESTATIC FooKt.doSomething (LFoo;)V
RETURN
L1
LOCALVARIABLE $receiver LFoo; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 1
Private Package Level Extension Function: Bytecode
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public final class FooKt {
private final static doSomething(LFoo;)V
…
public final static synthetic access$doSomething(LFoo;)V
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
LINENUMBER 1 L0
ALOAD 0
INVOKESTATIC FooKt.doSomething (LFoo;)V
RETURN
L1
LOCALVARIABLE $receiver LFoo; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 1
Private Package Level Extension Function: Bytecode
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Conclusion of Extension Function
Package Level Extension Function -> Static Method
Class Level Extension Function -> Method
Call Private Package Level Extension Function from inside a class of same file
-> Private Static Method and Accessor Method
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Named Argument
Default Argument
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fun foo(one: String, two: String, three: String) {
}
foo(three = "tres", one = "uno", two = "dos")
Named Argument: Kotlin
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LDC "tres"
LDC "uno"
LDC "dos"
ASTORE 1
ASTORE 2
ASTORE 3
ALOAD 2
ALOAD 1
ALOAD 3
INVOKEVIRTUAL Foo.foo (Ljava/lang/String;Ljava/lang/String;Ljava/lang/
String;)V
Named Argument: Bytecode
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LDC "tres"
LDC "uno"
LDC "dos"
ASTORE 1
ASTORE 2
ASTORE 3
ALOAD 2
ALOAD 1
ALOAD 3
INVOKEVIRTUAL Foo.foo (Ljava/lang/String;Ljava/lang/String;Ljava/lang/
String;)V
Named Argument: Bytecode
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LDC "tres"
LDC "uno"
LDC "dos"
ASTORE 1
ASTORE 2
ASTORE 3
ALOAD 2
ALOAD 1
ALOAD 3
INVOKEVIRTUAL Foo.foo (Ljava/lang/String;Ljava/lang/String;Ljava/lang/
String;)V
Named Argument: Bytecode
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LDC "tres"
LDC "uno"
LDC "dos"
ASTORE 1
ASTORE 2
ASTORE 3
ALOAD 2
ALOAD 1
ALOAD 3
INVOKEVIRTUAL Foo.foo (Ljava/lang/String;Ljava/lang/String;Ljava/lang/
String;)V
Named Argument: Bytecode
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String var1 = "dos";
String var2 = "uno";
String var3 = "tres";
this.foo(var2, var1, var3);
Named Argument: Java
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fun foo(one: String = “one",
two: String = “two",
three: String = "three") {
}
foo()
foo("uno")
foo("uno", "dos")
foo("uno", "dos", "tres")
foo(two = "dos")
Default Argument: Kotlin
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fun foo(one: String = “one",
two: String = “two",
three: String = "three") {
}
foo(two = "dos")
Default Argument: Kotlin
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‣ public static void foo(String, String, String)
‣ public static void foo$default(String, String, String, int, Object)
Default Argument: Java
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foo(two = "dos")
Default Argument: Kotlin
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
0
1 1
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
var1: null
var2: “dos”
var3: null
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101 001
101
and 001
001
var1: null
var2: “dos”
var3: null
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101 001
101
and 001
001
var1: “one”
var2: “dos”
var3: null
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101 010
101
and 010
000
var1: “one”
var2: “dos”
var3: null
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101 100
101
and 100
100
var1: “one”
var2: “dos”
var3: null
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
101 100
101
and 100
100
var1: “one”
var2: “dos”
var3: “three”
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public static void foo$default(
String var1, String var2, String var3, int var4, Object var5) {
if((var4 & 1) != 0) {
var1 = "one";
}
if((var4 & 2) != 0) {
var2 = "two";
}
if((var4 & 4) != 0) {
var3 = "three";
}
foo(var1, var2, var3);
}
foo$default((String)null, "dos", (String)null, 5, (Object)null);
Default Argument: Java
“one”“dos”“three”
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Thank you