Java方法返回类型深度解析：何时、为何以及如何返回类实例

public class ExampleMethods {
    // 返回void，不返回任何值
    public void printMessage(String message) {
        System.out.println(message);
    }

    // 返回int类型
    public int add(int a, int b) {
        return a + b;
    }

    // 返回String类型
    public String getGreeting() {
        return "Hello, Java!";
    }

    // 返回自定义类Bird的实例
    public Bird createBird() {
        return new Bird("Sparrow");
    }
}

class Bird {
    String species;
    public Bird(String species) {
        this.species = species;
    }
    public String getSpecies() {
        return species;
    }
}

public class Bird {
    String name;

    public Bird(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    // 示例方法：返回Bird类型的实例
    public static Bird createDefaultBird() {
        return new Bird("Robin"); // 返回一个Bird对象
    }

    // 示例方法：可能返回null
    public static Bird findBirdByName(String searchName) {
        if ("Eagle".equals(searchName)) {
            return new Bird("Eagle");
        }
        return null; // 如果找不到，返回null
    }
}

class ValidationResult {
    private boolean success;
    private String message;

    public ValidationResult(boolean success, String message) {
        this.success = success;
        this.message = message;
    }

    public boolean isSuccess() {
        return success;
    }

    public String getMessage() {
        return message;
    }
}

class AuthService {
    public ValidationResult validateCredentials(String username, String password) {
        if ("admin".equals(username) && "password123".equals(password)) {
            return new ValidationResult(true, "Authentication successful.");
        } else if ("admin".equals(username)) {
            return new ValidationResult(false, "Incorrect password.");
        } else {
            return new ValidationResult(false, "Username not found.");
        }
    }
}

// 使用示例
public class Main {
    public static void main(String[] args) {
        AuthService authService = new AuthService();
        ValidationResult result = authService.validateCredentials("admin", "wrong_pass");

        if (result.isSuccess()) {
            System.out.println("Login successful: " + result.getMessage());
        } else {
            System.out.println("Login failed: " + result.getMessage());
        }
    }
}

class Coffee {
    private String type;
    private boolean hasMilk;
    private boolean hasSugar;

    private Coffee(String type, boolean hasMilk, boolean hasSugar) {
        this.type = type;
        this.hasMilk = hasMilk;
        this.hasSugar = hasSugar;
    }

    @Override
    public String toString() {
        return "Coffee [type=" + type + ", hasMilk=" + hasMilk + ", hasSugar=" + hasSugar + "]";
    }

    // 静态内部类作为构建器
    public static class Builder {
        private String type;
        private boolean hasMilk = false; // 默认值
        private boolean hasSugar = false; // 默认值

        public Builder(String type) {
            this.type = type;
        }

        public Builder addMilk() {
            this.hasMilk = true;
            return this; // 返回当前Builder实例，实现链式调用
        }

        public Builder addSugar() {
            this.hasSugar = true;
            return this; // 返回当前Builder实例，实现链式调用
        }

        public Coffee build() {
            return new Coffee(type, hasMilk, hasSugar);
        }
    }
}

// 使用示例
public class Main {
    public static void main(String[] args) {
        Coffee latte = new Coffee.Builder("Latte")
                                 .addMilk()
                                 .build();
        System.out.println(latte); // Coffee [type=Latte, hasMilk=true, hasSugar=false]

        Coffee espressoWithSugar = new Coffee.Builder("Espresso")
                                             .addSugar()
                                             .build();
        System.out.println(espressoWithSugar); // Coffee [type=Espresso, hasMilk=false, hasSugar=true]
    }
}

interface Vehicle {
    void drive();
}

class Car implements Vehicle {
    @Override
    public void drive() {
        System.out.println("Driving a car.");
    }
}

class Motorcycle implements Vehicle {
    @Override
    public void drive() {
        System.out.println("Riding a motorcycle.");
    }
}

class VehicleFactory {
    public static Vehicle createVehicle(String type) {
        if ("car".equalsIgnoreCase(type)) {
            return new Car();
        } else if ("motorcycle".equalsIgnoreCase(type)) {
            return new Motorcycle();
        } else {
            throw new IllegalArgumentException("Unknown vehicle type: " + type);
        }
    }
}

// 使用示例
public class Main {
    public static void main(String[] args) {
        Vehicle myCar = VehicleFactory.createVehicle("car");
        myCar.drive(); // Driving a car.

        Vehicle myMotorcycle = VehicleFactory.createVehicle("motorcycle");
        myMotorcycle.drive(); // Riding a motorcycle.
    }
}