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# From Java To Kotlin
> From Java To Kotlin - Your Cheat Sheet For Java To Kotlin
## About me
Hi, I am [**Amit Shekhar**](https://amitshekhar.me), I have mentored many developers, and their efforts landed them high-paying tech jobs, helped many tech companies in solving their unique problems, and created many open-source libraries being used by top companies. I am passionate about sharing knowledge through open-source, blogs, and videos.
You can connect with me on:
- [Twitter](https://twitter.com/amitiitbhu)
- [YouTube](https://www.youtube.com/@amitshekhar)
- [LinkedIn](https://www.linkedin.com/in/amit-shekhar-iitbhu)
- [GitHub](https://github.com/amitshekhariitbhu)
## **Get Mentorship: [amitshekhar.me](https://amitshekhar.me)**
## Print to Console
> Java
```java
System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");
```
> Kotlin
```kotlin
print("Amit Shekhar")
println("Amit Shekhar")
```
---
## Constants and Variables
> Java
```java
String name = "Amit Shekhar";
final String name = "Amit Shekhar";
```
> Kotlin
```kotlin
var name = "Amit Shekhar"
val name = "Amit Shekhar"
```
---
## Assigning the null value
> Java
```java
String otherName;
otherName = null;
```
> Kotlin
```kotlin
var otherName : String?
otherName = null
```
---
## Verify if value is null
> Java
```java
if (text != null) {
int length = text.length();
}
```
> Kotlin
```kotlin
text?.let {
val length = text.length
}
// or simply
val length = text?.length
```
---
## Verify if value is NotNull OR NotEmpty
> Java
```java
String sampleString = "Shekhar";
if (!sampleString.isEmpty()) {
myTextView.setText(sampleString);
}
if(sampleString!=null && !sampleString.isEmpty()){
myTextView.setText(sampleString);
}
```
> Kotlin
```kotlin
var sampleString ="Shekhar"
if(sampleString.isNotEmpty()){ //the feature of kotlin extension function
myTextView.text=sampleString
}
if(!sampleString.isNullOrEmpty()){
myTextView.text=sampleString
}
```
---
## Concatenation of strings
> Java
```java
String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;
```
> Kotlin
```kotlin
var firstName = "Amit"
var lastName = "Shekhar"
var message = "My name is: $firstName $lastName"
```
---
## New line in string
> Java
```java
String text = "First Line\n" +
"Second Line\n" +
"Third Line";
```
> Kotlin
```kotlin
val text = """
|First Line
|Second Line
|Third Line
""".trimMargin()
```
---
## Substring
> Java
```java
String str = "Java to Kotlin Guide";
String substr = "";
//print java
substr = str.substring(0, 4);
System.out.println("substring = " + substr);
//print kotlin
substr = str.substring(8, 14);
System.out.println("substring = " + substr);
```
> Kotlin
```kotlin
var str = "Java to Kotlin Guide"
var substr = ""
//print java
substr = str.substring(0..3) //
println("substring $substr")
//print kotlin
substr = str.substring(8..13)
println("substring $substr")
```
---
## Ternary Operations
> Java
```java
String text = x > 5 ? "x > 5" : "x <= 5";
String message = null;
log(message != null ? message : "");
```
> Kotlin
```kotlin
val text = if (x > 5) "x > 5" else "x <= 5"
val message: String? = null
log(message ?: "")
```
---
## Bitwise Operators
> Java
```java
final int andResult = a & b;
final int orResult = a | b;
final int xorResult = a ^ b;
final int rightShift = a >> 2;
final int leftShift = a << 2;
final int unsignedRightShift = a >>> 2;
```
> Kotlin
```kotlin
val andResult = a and b
val orResult = a or b
val xorResult = a xor b
val rightShift = a shr 2
val leftShift = a shl 2
val unsignedRightShift = a ushr 2
```
---
## Check the type and casting
> Java
```java
if (object instanceof Car) {
Car car = (Car) object;
}
```
> Kotlin
```kotlin
if (object is Car) {
var car = object as Car
}
// if object is null
var car = object as? Car // var car = object as Car?
```
---
## Check the type and casting (implicit)
> Java
```java
if (object instanceof Car) {
Car car = (Car) object;
}
```
> Kotlin
```kotlin
if (object is Car) {
var car = object // smart casting
}
// if object is null
if (object is Car?) {
var car = object // smart casting, car will be null
}
```
---
## Multiple conditions
> Java
```java
if (score >= 0 && score <= 300) { }
```
> Kotlin
```kotlin
if (score in 0..300) { }
```
---
## Multiple Conditions (Switch case)
> Java
```java
int score = // some score;
String grade;
switch (score) {
case 10:
case 9:
grade = "Excellent";
break;
case 8:
case 7:
case 6:
grade = "Good";
break;
case 5:
case 4:
grade = "OK";
break;
case 3:
case 2:
case 1:
grade = "Fail";
break;
default:
grade = "Fail";
}
```
> Kotlin
```kotlin
var score = // some score
var grade = when (score) {
9, 10 -> "Excellent"
in 6..8 -> "Good"
4, 5 -> "OK"
else -> "Fail"
}
```
---
## For-loops
> Java
```java
for (int i = 1; i <= 10 ; i++) { }
for (int i = 1; i < 10 ; i++) { }
for (int i = 10; i >= 0 ; i--) { }
for (int i = 1; i <= 10 ; i+=2) { }
for (int i = 10; i >= 0 ; i-=2) { }
for (String item : collection) { }
for (Map.Entry<String, String> entry: map.entrySet()) { }
```
> Kotlin
```kotlin
for (i in 1..10) { }
for (i in 1 until 10) { }
for (i in 10 downTo 0) { }
for (i in 1..10 step 2) { }
for (i in 10 downTo 0 step 2) { }
for (item in collection) { }
for ((key, value) in map) { }
```
---
## Collections
> Java
```java
final List<Integer> listOfNumber = Arrays.asList(1, 2, 3, 4);
final Map<Integer, String> keyValue = new HashMap<Integer, String>();
map.put(1, "Amit");
map.put(2, "Anand");
map.put(3, "Messi");
// Java 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);
final Map<Integer, String> keyValue = Map.of(1, "Amit",
2, "Anand",
3, "Messi");
```
> Kotlin
```kotlin
val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Amit",
2 to "Anand",
3 to "Messi")
```
---
## for each
> Java
```java
// Java 7 and below
for (Car car : cars) {
System.out.println(car.speed);
}
// Java 8+
cars.forEach(car -> System.out.println(car.speed));
// Java 7 and below
for (Car car : cars) {
if (car.speed > 100) {
System.out.println(car.speed);
}
}
// Java 8+
cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));
cars.parallelStream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));
```
> Kotlin
```kotlin
cars.forEach {
println(it.speed)
}
cars.filter { it.speed > 100 }
.forEach { println(it.speed)}
// kotlin 1.1+
cars.stream().filter { it.speed > 100 }.forEach { println(it.speed)}
cars.parallelStream().filter { it.speed > 100 }.forEach { println(it.speed)}
```
---
## Splitting arrays
> java
```java
String[] splits = "param=car".split("=");
String param = splits[0];
String value = splits[1];
```
> kotlin
```kotlin
val (param, value) = "param=car".split("=")
```
---
## Defining methods
> Java
```java
void doSomething() {
// logic here
}
```
> Kotlin
```kotlin
fun doSomething() {
// logic here
}
```
### Default values for method parameters
> Java
```java
double calculateCost(int quantity, double pricePerItem) {
return pricePerItem * quantity;
}
double calculateCost(int quantity) {
// default price is 20.5
return 20.5 * quantity;
}
```
> Kotlin
```kotlin
fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItem
calculateCost(10, 25.0) // 250
calculateCost(10) // 205
```
---
## Variable number of arguments
> Java
```java
void doSomething(int... numbers) {
// logic here
}
```
> Kotlin
```kotlin
fun doS