Structs vs Classes - Java to C | codemirr

Introduction

In this lesson, you'll learn about structs vs classes in C. Coming from Java, you already have a foundation for understanding this concept. We'll build on that knowledge while highlighting the key differences.

Mirror Card

From Java:

In Java, you're familiar with data structures in c.

In C:

C has its own approach to data structures in c, which we'll explore step by step.

The C Way

Let's see how C handles this concept. Here's a typical example:

C Example

#include <stdio.h>
#include <string.h>

// C struct - data only, no methods
typedef struct {
    char name[50];
    int age;
} Person;

// "Constructor" function
Person* createPerson(const char* name, int age) {
    Person* p = malloc(sizeof(Person));
    strcpy(p->name, name);
    p->age = age;
    return p;
}

// "Method" - takes struct pointer
void birthday(Person* p) {
    p->age++;
}

void printPerson(Person* p) {
    printf("%s, %d\n", p->name, p->age);
}

// Usage
Person* p = createPerson("Alice", 30);
birthday(p);
printPerson(p);
free(p);

Comparing to Java

Here's how you might have written similar code in Java:

Java (What you know)

// Java class with methods
public class Person {
    private String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public void birthday() {
        this.age++;
    }
    
    public String toString() {
        return name + ", " + age;
    }
}

Person p = new Person("Alice", 30);
p.birthday();

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

Structs only contain data, not methods

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

typedef creates a type alias for convenience

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

"Methods" are functions that take struct pointer

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

No access modifiers (public/private)

Step-by-Step Breakdown

1. Struct Definition

Structs group related data together. Use typedef for cleaner syntax.

// Without typedef:
struct Person {
    char name[50];
    int age;
};
struct Person p1; // must use 'struct' keyword

// With typedef:
typedef struct {
    char name[50];
    int age;
} Person;
Person p2; // cleaner!

2. Simulating Methods

Pass the struct (usually as a pointer) as the first parameter to functions.

typedef struct {
    int x, y;
} Point;

// "Methods" take pointer to struct
void point_move(Point* self, int dx, int dy) {
    self->x += dx;
    self->y += dy;
}

double point_distance(Point* self, Point* other) {
    int dx = self->x - other->x;
    int dy = self->y - other->y;
    return sqrt(dx*dx + dy*dy);
}

// Usage
Point p = {0, 0};
point_move(&p, 10, 20);

Rule of Thumb

Name functions with a prefix matching the struct: point_move, point_distance. This simulates namespacing.

3. Nested Structs

Structs can contain other structs or pointers to structs.

typedef struct {
    char street[100];
    char city[50];
} Address;

typedef struct {
    char name[50];
    Address address;      // embedded struct
    Address* workAddr;    // pointer to struct
} Employee;

Employee e;
strcpy(e.address.city, "NYC"); // access nested
e.workAddr = malloc(sizeof(Address));
strcpy(e.workAddr->city, "LA"); // via pointer

Common Mistakes

When coming from Java, developers often make these mistakes:

Structs only contain data, not methods
typedef creates a type alias for convenience
"Methods" are functions that take struct pointer

Common Pitfall

Don't assume C works exactly like Java. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Structs are data containers without methods
Use typedef for cleaner type names
Functions taking struct* simulate methods
No encapsulation - all fields are accessible

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your Java code in C to practice these concepts.

Introduction

Mirror Card

From Java:

In Java, you're familiar with data structures in c.

In C:

C has its own approach to data structures in c, which we'll explore step by step.

The C Way

Let's see how C handles this concept. Here's a typical example:

C Example

#include <stdio.h>
#include <string.h>

// C struct - data only, no methods
typedef struct {
    char name[50];
    int age;
} Person;

// "Constructor" function
Person* createPerson(const char* name, int age) {
    Person* p = malloc(sizeof(Person));
    strcpy(p->name, name);
    p->age = age;
    return p;
}

// "Method" - takes struct pointer
void birthday(Person* p) {
    p->age++;
}

void printPerson(Person* p) {
    printf("%s, %d\n", p->name, p->age);
}

// Usage
Person* p = createPerson("Alice", 30);
birthday(p);
printPerson(p);
free(p);

Comparing to Java

Here's how you might have written similar code in Java:

Java (What you know)

// Java class with methods
public class Person {
    private String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public void birthday() {
        this.age++;
    }
    
    public String toString() {
        return name + ", " + age;
    }
}

Person p = new Person("Alice", 30);
p.birthday();

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

Structs only contain data, not methods

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

typedef creates a type alias for convenience

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

"Methods" are functions that take struct pointer

Mirror Card

From Java:

You may be used to different syntax or behavior.

In C:

No access modifiers (public/private)

Step-by-Step Breakdown

1. Struct Definition

Structs group related data together. Use typedef for cleaner syntax.

// Without typedef:
struct Person {
    char name[50];
    int age;
};
struct Person p1; // must use 'struct' keyword

// With typedef:
typedef struct {
    char name[50];
    int age;
} Person;
Person p2; // cleaner!

2. Simulating Methods

Pass the struct (usually as a pointer) as the first parameter to functions.

typedef struct {
    int x, y;
} Point;

// "Methods" take pointer to struct
void point_move(Point* self, int dx, int dy) {
    self->x += dx;
    self->y += dy;
}

double point_distance(Point* self, Point* other) {
    int dx = self->x - other->x;
    int dy = self->y - other->y;
    return sqrt(dx*dx + dy*dy);
}

// Usage
Point p = {0, 0};
point_move(&p, 10, 20);

Rule of Thumb

Name functions with a prefix matching the struct: point_move, point_distance. This simulates namespacing.

3. Nested Structs

Structs can contain other structs or pointers to structs.

typedef struct {
    char street[100];
    char city[50];
} Address;

typedef struct {
    char name[50];
    Address address;      // embedded struct
    Address* workAddr;    // pointer to struct
} Employee;

Employee e;
strcpy(e.address.city, "NYC"); // access nested
e.workAddr = malloc(sizeof(Address));
strcpy(e.workAddr->city, "LA"); // via pointer

Common Mistakes

When coming from Java, developers often make these mistakes:

Structs only contain data, not methods
typedef creates a type alias for convenience
"Methods" are functions that take struct pointer

Common Pitfall

Don't assume C works exactly like Java. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Structs are data containers without methods
Use typedef for cleaner type names
Functions taking struct* simulate methods
No encapsulation - all fields are accessible

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your Java code in C to practice these concepts.