Java三种对象访问方式示例请求：组合、参数传递、局部声明

Hey there! Let's walk through all three ways to work with objects in Java you're learning, with complete, runnable examples. First, let's start with a basic Temperature class since that's what you're using—this will be the foundation for all our examples:

public class Temperature {
    private double value;
    private char scale;

    // Constructor you referenced
    public Temperature(double value, char scale) {
        this.value = value;
        this.scale = scale;
    }

    // Getters to access the values (useful for our examples)
    public double getValue() {
        return value;
    }

    public char getScale() {
        return scale;
    }

    // Optional: Method to convert to Celsius for demonstration
    public double toCelsius() {
        if (scale == 'F') {
            return (value - 32) * 5/9;
        }
        return value; // Already Celsius
    }
}

Think of this as a "has-a" relationship—one object owns another as a member variable. For example, a thermostat device logically needs a temperature reading, so we can build that ownership into the class:

public class Thermostat {
    // This is the composed object—Thermostat "has a" Temperature
    private Temperature currentTemp;

    // Constructor that initializes the internal Temperature
    public Thermostat(double initialValue, char initialScale) {
        this.currentTemp = new Temperature(initialValue, initialScale);
    }

    // Update the stored temperature
    public void updateTemperature(double newValue, char newScale) {
        this.currentTemp = new Temperature(newValue, newScale);
    }

    // Display the current temp converted to Celsius
    public void displayCurrentCelsius() {
        System.out.printf("Current temperature: %.2f°C%n", currentTemp.toCelsius());
    }

    public static void main(String[] args) {
        // Create a Thermostat, which internally creates a Temperature object
        Thermostat livingRoomThermo = new Thermostat(72.0, 'F');
        livingRoomThermo.displayCurrentCelsius(); // Outputs ~22.22°C

        // Update the temperature (replaces the internal Temperature object)
        livingRoomThermo.updateTemperature(25.0, 'C');
        livingRoomThermo.displayCurrentCelsius(); // Outputs 25.00°C
    }
}

You already started this with instantiating a Temperature object—let's expand it into a full example where we pass that object to a method to perform an action on it:

public class TemperatureConverter {
    // Method that accepts a Temperature object as input
    public static void printTemperatureInFahrenheit(Temperature temp) {
        double fahrenheitValue;
        if (temp.getScale() == 'C') {
            fahrenheitValue = (temp.getValue() * 9/5) + 32;
        } else {
            fahrenheitValue = temp.getValue();
        }
        System.out.printf("Temperature in Fahrenheit: %.2f°F%n", fahrenheitValue);
    }

    public static void main(String[] args) {
        // Your original instantiation
        Temperature tempsF = new Temperature(32.6, 'F');
        // Pass the object to our method
        printTemperatureInFahrenheit(tempsF); // Outputs 32.60°F

        // Test with a Celsius temperature too
        Temperature tempsC = new Temperature(0.0, 'C');
        printTemperatureInFahrenheit(tempsC); // Outputs 32.00°F
    }
}

This is when you create an object only within the scope of a method—it doesn't exist outside that method. Perfect for objects that are only needed temporarily for a specific task:

public class TempChecker {
    public static void checkIfFreezing() {
        // Local object—only exists inside this method
        Temperature outdoorTemp = new Temperature(31.0, 'F');

        if (outdoorTemp.toCelsius() < 0) {
            System.out.println("It's freezing outside!");
        } else {
            System.out.println("Above freezing.");
        }
    }

    public static void main(String[] args) {
        checkIfFreezing(); // Outputs "It's freezing outside!"

        // Try uncommenting the line below—you'll get a compile error!
        // System.out.println(outdoorTemp.getValue());
        // Because outdoorTemp is local to checkIfFreezing() and not visible here
    }
}

内容的提问来源于stack exchange，提问作者B.Allen