Classes in scala (decompiled to Java byte code to see generated methods)
scala> me@MacBook:~$ scala
Welcome to Scala 2.11.12 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_192).
Type in expressions for evaluation. Or try :help.
scala> class Person(name: String, age: Int) {
| println("In default constructor")
| override def toString = "Person(name=" + name + ",age=" + age + ")"
| }
defined class Person
scala> new Person("Bob",18)
In default constructor
res0: Person = Person(name=Bob,age=18)
scala> :javap -p Person
Compiled from ""
public class Person {
private final java.lang.String name;
private final int age;
public java.lang.String toString();
public Person(java.lang.String, int);
}
scala> :paste
// Entering paste mode (ctrl-D to finish)
class Person(name: String, age: Int) {
override def toString = "Person(name=" + name + ",age=" + age + ")"
}
object Person {
def apply(name: String, age: Int) = { // create factory method(s) using apply method(s)
new Person(name,age)
}
}
// Exiting paste mode, now interpreting.
defined class Person
defined object Person // companion object
scala> new Person("Alice",19)
res0: Person = Person(name=Alice,age=19)
scala> Person.apply("Alice", 19) // access companion object methods by object-name.method-name
res1: Person = Person(name=Alice,age=19)
scala> Person("Alice", 19) // can even skip typing apply method
res2: Person = Person(name=Alice,age=19)
scala> :javap -p Person$
Compiled from ""
public class Person$ {
public static final Person$ MODULE$;
public static {};
public Person apply(java.lang.String, int);
public Person$();
}
scala> class Person(val name: String, val age: Int) {
| println("In default constructor")
| override def toString = "Person(name=" + name + ",age=" + age + ")"
| }
defined class Person
scala> :javap -p Person
Compiled from ""
public class Person {
private final java.lang.String name;
private final int age;
public java.lang.String name();
public int age();
public java.lang.String toString();
public Person(java.lang.String, int);
}
scala> new Person("Bob",18)
In default constructor
res1: Person = Person(name=Bob,age=18)
scala> res1.name
res2: String = Bob
scala> res1.name()
:13: error: not enough arguments for method apply: (index: Int)Char in class StringOps.
Unspecified value parameter index.
res1.name()
^
scala> class Person(var name: String, var age: Int) {
| override def toString = "Person(name=" + name + ",age=" + age + ")"
| }
defined class Person
scala> :javap -p Person
Compiled from ""
public class Person {
private java.lang.String name;
private int age;
public java.lang.String name();
public void name_$eq(java.lang.String);
public int age();
public void age_$eq(int);
public java.lang.String toString();
public Person(java.lang.String, int);
}
scala> new Person("Bob",18)
res4: Person = Person(name=Bob,age=18)
scala> res4.name_=("Bobby")
scala> res4
res6: Person = Person(name=Bobby,age=18)
scala> res4.name = "Bobby2"
res4.name: String = Bobby2
scala> res4
res7: Person = Person(name=Bobby2,age=18)
scala> case class Person(var name: String, var age: Int) // var to add setter methods
defined class Person
scala> new Person("Alice",17)
res0: Person = Person(Alice,17) // case class has by default toString with fields (instead byte address)
scala> Person("Bobby",18) // automatically added companion object with apply method
res1: Person = Person(Bobby,18)
scala> :javap -p Person
Compiled from ""
public class Person implements scala.Product,scala.Serializable {
private java.lang.String name;
private int age;
public java.lang.String name();
public void name_$eq(java.lang.String); // var to add setter methods
public int age();
public void age_$eq(int);
public Person copy(java.lang.String, int);
public java.lang.String copy$default$1();
public int copy$default$2();
public java.lang.String productPrefix();
public int productArity();
public java.lang.Object productElement(int);
public scala.collection.Iterator productIterator();
public boolean canEqual(java.lang.Object);
public int hashCode();
public java.lang.String toString();
public boolean equals(java.lang.Object);
public Person(java.lang.String, int);
}
scala> :javap -p scala.Serializable
Compiled from "Serializable.scala"
public interface scala.Serializable extends java.io.Serializable {
}
scala> :javap -p java.io.Serializable
Compiled from "Serializable.java"
public interface java.io.Serializable {
}
scala> :javap -p scala.Product
Compiled from "Product.scala"
public interface scala.Product extends scala.Equals {
public abstract java.lang.Object productElement(int);
public abstract int productArity();
public abstract scala.collection.Iterator productIterator();
public abstract java.lang.String productPrefix();
}
scala> :javap -p Person$ // automatically added companion object with apply method
Compiled from ""
public class Person$ extends scala.runtime.AbstractFunction2 implements scala.Serializable {
public static final Person$ MODULE$;
public static {};
public final java.lang.String toString();
public Person apply(java.lang.String, int);
public scala.Option<scala.Tuple2> unapply(Person);
private java.lang.Object readResolve();
public java.lang.Object apply(java.lang.Object, java.lang.Object);
public Person$();
}
scala> case class Person(name: String, age: Int) // no var before args - no setters generated
defined class Person
scala> new Person("Alice",17)
res0: Person = Person(Alice,17)
scala> Person("Bobby",18)
res1: Person = Person(Bobby,18)
scala> :javap -p Person
Compiled from ""
public class Person implements scala.Product,scala.Serializable {
private final java.lang.String name;
private final int age;
public java.lang.String name();
public int age();
public Person copy(java.lang.String, int);
public java.lang.String copy$default$1();
public int copy$default$2();
public java.lang.String productPrefix();
public int productArity();
public java.lang.Object productElement(int);
public scala.collection.Iterator productIterator();
public boolean canEqual(java.lang.Object);
public int hashCode();
public java.lang.String toString();
public boolean equals(java.lang.Object);
public Person(java.lang.String, int);
}
scala> :javap -p Person$
Compiled from ""
public class Person$ extends scala.runtime.AbstractFunction2 implements scala.Serializable {
public static final Person$ MODULE$;
public static {};
public final java.lang.String toString();
public Person apply(java.lang.String, int);
public scala.Option<scala.Tuple2> unapply(Person);
private java.lang.Object readResolve();
public java.lang.Object apply(java.lang.Object, java.lang.Object);
public Person$();
}
scala> case class Person(name: String, age: Int) { // override constructor and toString method in case class
| println("In my constructor")
| override def toString = name + age
| }
defined class Person
scala> new Person("a",1)
In my constructor
res4: Person = a1
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