Does Java support structs?

Question

Does Java have an analog of a C++ struct:

struct Member {
  string FirstName; 
  string LastName;  
  int BirthYear; 
};

I need to use my own data type.

Prophetic

For those copying too literally: fields in Java really need to start with a lowercase character, so firstName, lastName and birthYear (or yearOfBirth of course). – Intelligentsia

Please select this answer regarding the upcoming "inline classes" as the answer. AFAICT, all the other answers are using variants on regular objects. – Gorrono

Answer 1

Java definitively has no structs :) But what you describe here looks like a JavaBean kind of class.

Biology

Well, a class with only public variables looks awfully like a stuct. – Orthohydrogen

We will have records 9 years after this answer, in Java 14. It is amusing how fast time flies. – Karlin

One of the major differences between classes and structs is how they behave in memory. In this respect, this answer is wrong. Java has nothing that behaves like a struct but will hopefully have inline class at some point in the near future which I believe is more like a struct. – Camelback

A bean has properties (i.e. getters and usually setters) and nowadays commonly is annotated for it to be comply with the requirements for a bean. – Intelligentsia

Struct is very good thing for questions in interview. I have an enterprise product, no using struct. C# 7.0 – Duodiode

@WillCalderwood, bingo. I've seen it credited as one of the biggest reasons that optimized C# code outperforms optimized Java. I think the problem comes from an entire generation of engineers that have never known a compound value type before such as C has. They think in terms of everything either being a pointer (ref) to something or a primitive. In fact, the Valhalla guys have changed their minds regarding names an "inline classes" (etc.) may well be "primitive classes". I just wish they'd merge the thing, because Structs and Reified Generics are pretty important. – Gorrono

Answer 2

The equivalent in Java to a struct would be

class Member
{
    public String firstName;
    public String lastName;
    public int    birthYear;
 };

and there's nothing wrong with that in the right circumstances. Much the same as in C++ really in terms of when do you use struct versus when do you use a class with encapsulated data.

Andrus

I'd disagree - it takes little effort to refactor such a class into a "proper" class with getters and setters using modern IDEs, and this provides a much more robust and extensible framework, whatever the use (academic reasons aside!) – Sandi

I would use "POD" type classes in java in the same circumstances as I do in C++. "Value objects" spring to mind in J2EE where you have a method that is returning multiple values at once. No point in encapsulating the data with getter and setter methods, just make the fields public -- shorter and more descriptive. Anything "proper" I tend to encapsulate, in the same was I do in C++. So I stand by my original comment, but it's a style choice. I don't think you loose anything, and indeed I think it's more descriptive when what you've got is a value class. – Andrus

No need to end a class in java with a semicolon. – Stolid

Correct me if I'm wrong, but the point of Struct is that it is passed by value not by reference. Classes always are passed by reference. That's why this answer is wrong. – Sensualist

@ULazdins: I think you're confusing C# structs with C++ structs. When you pass a class to a method in C++, it calls the copy constructor, which by default copies each field. You have to explicitly use the syntax to pass it by reference. – Feint

I wanted to say that I totally disagree with @MichaelBerry. If you want a lot of redundant code to get an unmaintainable framework, waste your time turning all your structs into classes with private members and accessors. – Sidra

@PabloAriel I'd also disagree with my earlier comment. My thinking is allowed to evolve over 7 years ;) – Sandi

What about garbage collection? I mean does this produce overhead for garbage collector? – Mollie

@MichaelBerry Happy to hear that. I have to admit I didn't think as I do now back in 2011. – Sidra

please format your Java code properly (camelCasing, spaces, etc) – Vanguard

@RonaldoNazarea, ULazdins is right, C# structs are value types. see learn.microsoft.com/en-us/dotnet/csharp/programming-guide/… – Katharinekatharsis

Probably there is something wrong with the code if the fields are made public, because that indicates that the struct can be used outside of the package that defines it. Just leaving it to package access (no modifier) should be preferred. And if possible, use it as static inner class and mark the class itself private. – Intelligentsia

As others said: NOT REALLY STRUCTS, structs as in C++ are much easier, simple, compact (no need for different files). Very sorry java don't have then... – Victimize

@Victimize You can define nested classes in Java just fine. It just doesn't let you switch "topics" in the same "document." One of Java's primary design goals (IIRC) was to distinguish itself from C++, particularly by enforcing certain style guidelines at the language level. The idea was, you can still write bad programs in Java, it's just not as easy. Since then, Java's specific style guidelines may seem a bit dated to me, but... it was a decent idea. – Dubonnet

I'm beginning to wonder if some of the folks even know what a struct truly is, or used a language that had them, such as C. If you've only been on language platforms like (pre-Valhalla) Java where every last thing is either a reference to something or a primitive, it's going to be confusing to you to think of a compound value type. – Gorrono

Answer 3

Java 14 has added support for Records, which are structured data types that are very easy to build.

You can declare a Java record like this:

public record AuditInfo(
    LocalDateTime createdOn,
    String createdBy,
    LocalDateTime updatedOn,
    String updatedBy
) {}
 
public record PostInfo(
    Long id,
    String title,
    AuditInfo auditInfo
) {}

And, the Java compiler will generate the following Java class associated to the AuditInfo Record:

public final class PostInfo
        extends java.lang.Record {
    private final java.lang.Long id;
    private final java.lang.String title;
    private final AuditInfo auditInfo;
 
    public PostInfo(
            java.lang.Long id,
            java.lang.String title,
            AuditInfo auditInfo) {
        /* compiled code */
    }
 
    public java.lang.String toString() { /* compiled code */ }
 
    public final int hashCode() { /* compiled code */ }
 
    public final boolean equals(java.lang.Object o) { /* compiled code */ }
 
    public java.lang.Long id() { /* compiled code */ }
 
    public java.lang.String title() { /* compiled code */ }
 
    public AuditInfo auditInfo() { /* compiled code */ }
}
 
public final class AuditInfo
        extends java.lang.Record {
    private final java.time.LocalDateTime createdOn;
    private final java.lang.String createdBy;
    private final java.time.LocalDateTime updatedOn;
    private final java.lang.String updatedBy;
 
    public AuditInfo(
            java.time.LocalDateTime createdOn,
            java.lang.String createdBy,
            java.time.LocalDateTime updatedOn,
            java.lang.String updatedBy) {
        /* compiled code */
    }
 
    public java.lang.String toString() { /* compiled code */ }
 
    public final int hashCode() { /* compiled code */ }
 
    public final boolean equals(java.lang.Object o) { /* compiled code */ }
 
    public java.time.LocalDateTime createdOn() { /* compiled code */ }
 
    public java.lang.String createdBy() { /* compiled code */ }
 
    public java.time.LocalDateTime updatedOn() { /* compiled code */ }
 
    public java.lang.String updatedBy() { /* compiled code */ }
}

Notice that the constructor, accessor methods, as well as equals, hashCode, and toString are created for you, so it's very convenient to use Java Records.

A Java Record can be created like any other Java object:

PostInfo postInfo = new PostInfo(
    1L,
    "High-Performance Java Persistence",
    new AuditInfo(
        LocalDateTime.of(2016, 11, 2, 12, 0, 0),
        "Vlad Mihalcea",
        LocalDateTime.now(),
        "Vlad Mihalcea"
    )
);

Leventhal

This completely bypasses the entire point of structs in the first place (a compound datatype that doesn't need "allocation" and can live on the stack directly without a reference.) Please see this answer https://mcmap.net/q/166461/-does-java-support-structs regarding the upcoming "inline classes". – Gorrono

A record is nothing more than the same class/obj paradigm. It doesn't behave anything like a struct does. If you pass it, you're passing the reference by value, not the values by value, and you're still under the whimsy of the garbage collector. Until Valhalla solidifies, the closest the OP can get visually is a regular class with public fields. Not a fancy class with autogenerated accessors, etc. There is no need for accessors in a struct. – Gorrono

Your sense of "never" aside Vlad, what I'm saying is that Records provide nothing in approximating a struct that a simple class already gives you. A record is immutable (shallowly), structs/classes are not. A record does not answer the OP's question...it's a different technique. Records are in no way a solution to someone wanting structs unless we specify additional criteria (immutability, accessors, etc.). – Gorrono

do you have any experience with C or C++? The objects created by C++ are passed by default as value types. That means a copy is involved. Something you can't do in Java (yet), not even with records. Iit really seems that you've been exposed only to reference-mandated compound types. But a record only muddies the waters. If you want a struct right now (without Valhalla in the compiler + JVM), then a plain class instantiating plain objects will be closest. A java record doesn't aid the OP in any way at all. – Gorrono

And where did I say that in C++ they live only on the stack? I'm talking about the fundamental differences here. A record is not a value type. Make a struct in C++. Go do it. Instantiate it. Send it to a method (by value). modify one of the contents of the struct inside that method. Does it effect the outer value? No. Further, because of value types, the compiler also will substitute a.b (no matter where it exists) with a direct access to b via offset. You have no need to dereference a. Look up the design decision in C# to include structs and why it speeds up algorithms – Gorrono

Who ever said that Java will be C++? No one. And the OP brought in a facility that Java doesn't support, hence talking about what that facility actually is. And keeping it simple is precisely why you avoid Records and just go for a simple class. Anyway, we're clearly going around in circles, and you're content to argue against strawmen I never brought up. Yes, a C++ struct is essentially a C++ class, but no, it's nothing like a java class and even less like a java record. So use a class. – Gorrono

Answer 4

Actually a struct in C++ is a class (e.g. you can define methods there, it can be extended, it works exactly like a class), the only difference is that the default access modfiers are set to public (for classes they are set to private by default).

This is really the only difference in C++, many people don't know that. ; )

Botticelli

Answer 5

No, Java doesn't have struct/value type yet. But, in the upcoming version of Java, we are going to get inline class which is similar to struct in C# and will help us write allocation free code.

inline class point { 
  int x;
  int y;
}

Neurocoele

whoever downvoted this, may I know why? You can read more about java inline class here infoq.com/articles/inline-classes-java – Neurocoele

This is excellent! I look forward to the addition. – Norton

THIS is the answer. The others completely skirt the issue of having allocation/GC in the way of everything. Most importantly would be the advent of these things on the stack (local/passed/returned values) (if any of that is allowed, God willing.) It was my understanding that the JVM would need to be overhauled to allow this. So if this really did pan out, it's huge. – Gorrono

Looking into this: Can we clarify? In your example, x and y themselves are placed onto the stack if passed? That is, instead of a reference being passed by value, we have the values being passed by value like way C allows? – Gorrono

Further, Oracle seems to be a little self-argumentative regarding the term. mail.openjdk.java.net/pipermail/valhalla-spec-experts/… This is as of the end of 2020. I'm still not sure where Valhalla ended up. Anyway, "Primitive Objects", "Primitive Classes", "Primitive Value Types", etc.... might be the new terminology. – Gorrono

@Gorrono in C# struct objects are pass by value, hence I'm expecting point object will be pass by value, hence if the value changed in caller that would not change the callee's object at all. – Neurocoele

@ganesanarunachalam, "PBV" gets us into some trouble. To be clear, without the ref keyword, C# passes everything by value (references included). You meant struct objects contents are passed as values (as with C). Yes, I've done a lot of digging into Valhalla since I originally asked: The JVM is indeed modified to allow compound value types on the stack. The JUnion project seems like a weird half-stab that I haven't looked too deeply into. – Gorrono

What are the versions of Java in which this feature is supported? – Flagitious

openjdk.org/jeps/401 they are still in preview. @Flagitious – Neurocoele

Answer 6

Java doesn't have an analog to C++'s structs, but you can use classes with all public members.

Ginglymus

Actually a struct in C++ is (pretty much) the same thing as a class in C++. – Downtrodden

@JoachimSauer Weird! That makes me wonder how different C++ classes are from C structs. – Dubonnet

@MaartenBodewes The problem with internal is that it's less visible than protected. An internal member declared in com.myapp.lib will not be accessible in com.myapp.main, so you might not be able to access it everywhere within the same project. – Dubonnet

@jpaugh: I'm no C or C++ expert, that was just a random piece of interesting trivia that I picked up. But I guess since C++ classes are almost the same as C++ structs, I guess they are both quite different from C structs (which don't directly support inheritance of associated constructors/methods). – Downtrodden

@Dubonnet The whole reason I mentioned the issue with public is that you may not want to expose a record that can easily be put in a wrong state to many classes; it should preferably be kept internal. Note that Java records, as mentioned in this answer are immutable (but you can always use a factory method to create new records of course) – Intelligentsia

it is not the same, in the jvm an object is a pointer with other pointers for its fields, while a struct in c is a blueprint for a memory allocation so it can be easily managed since is always the same size and it is allogated one beside the others, like an array of int – Sankhya

Answer 7

With Project JUnion you can use structs in Java by annotating a class with @Struct annotation

@Struct
class Member {
  string FirstName; 
  string LastName;  
  int BirthYear; 
}

More info at the project's website: https://tehleo.github.io/junion/

Heterogenous

Good stuff. I now understand how structures (objects) are represented in memory. Thanks man :) – Alleyn

Answer 8

Yes, a class is what you need. An class defines an own type.

Eumenides

Answer 9

Along with Java 14, it starts supporting Record. You may want to check that https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/lang/Record.html

public record Person (String name, String address) {}

Person person = new Person("Esteban", "Stormhaven, Tamriel");

And there are Sealed Classes after Java 15. https://openjdk.java.net/jeps/360

sealed interface Shape permits Circle, Rectangle {

  record Circle(Point center, int radius) implements Shape { }

  record Rectangle(Point lowerLeft, Point upperRight) implements Shape { } 
}

Zacatecas

Is there any way you could give an example of a Record? – Haggle

Please don't just post some tool or library as an answer. At least demonstrate how it solves the problem in the answer itself. – Parmesan

Sorry just received the notification. Thought it is straightforward, but you are right. Added some examples. – Zacatecas

Records are still not structs. You're still munging around with an object, and cannot pass and return compound objects (structs) like C# can. Please see this answer regarding the upcoming "inline classes" support by ganesan arunachalam: https://mcmap.net/q/166461/-does-java-support-structs How it shakes out will be truly interesting. – Gorrono

Answer 10

Structs "really" pure aren't supported in Java. E.g., C# supports struct definitions that represent values and can be allocated anytime.

In Java, the unique way to get an approximation of C++ structs

struct Token
{
    TokenType type;
    Stringp stringValue;
    double mathValue;
}

// Instantiation

{
    Token t = new Token;
}

without using a (static buffer or list) is doing something like

var type = /* TokenType */ ;
var stringValue = /* String */ ;
var mathValue = /* double */ ;

So, simply allocate variables or statically define them into a class.

Brinkmanship

Java doesn't support var, does it? – Dubonnet

it is supported from java 10 as a basic placeholder for variables, since it is the constructor that will define the type, but I don't know if is usable for memory management, I think is better defining a variable for it's type, using a boolean it should be better that using var and then assign it to a boolean – Sankhya

Man, type inference of variables in C++ is something I miss, for sure. Glad to see it's made its way into Java. – Norton

@Dubonnet Note that var only cause the compiler to infer the type at compile-time, at runtime, there is no difference. – Jennee

I still don't understand where you're going with this answer. var does nothing to create a compound value type, which is the point behind structs. Further, what does putting them into a class accomplish? Are you answering his need for his own type, or answering what/why/how for structs? If the latter, this answer doesn't make sense. – Gorrono

Answer 11

The Immutables library provides something similar to what you are describing.

From their site:

import org.immutables.value.Value;
// Define abstract value type
@Value.Immutable
public interface ValueObject {
  String name();
  List<Integer> counts();
  Optional<String> description();
}

// Use generated immutable implementation
ValueObject valueObject =
    ImmutableValueObject.builder()
        .name("My value")
        .addCounts(1)
        .addCounts(2)
        .build();

Fredela

Answer 12

Since JAVA version 16 record was included. Record is practically the same as struct in C/C++. Syntax:

record name (field1, field2, ..., fieldN){
    // record body
}

Squint

Answer 13

The short answer: NO.

The long answer:

1. The main different between class and struct (in C++) is all properties in struct is public, which can be accessed from anywhere. For the class, you can apply limit it with different level of privacy.
2. If you wanna have a data structure same as struct in C++, just make all properties public.

Marianmariana

Comparisons to C++ are.....tricky. Contrast with C# instead --The reason that structures often speed up computation in C# over Java is that small structures can be placed onto the stack (local or passed, and of course returned). Not references to structures, but the data itself. This results in less mucking around with allocation, and even more so, doesn't impact the garbage collector. Anything that Java bolts onto the existing JVM will only be sugar coating that very heap-allocate/reference/GC process. The JVM specification itself needs to be modified to accommodate this. – Gorrono