按值调用：
调用的是实际参数或数值的副本（数据副本），改变的也只是副本的值，而实参的值不会改变
按引用调用：
1、初始化实参的一个副本
2、基于这个副本，方法可以改变原实参对象的参数值
3、方法结束后，参数变量（副本）不再使用，但改变了实参对象的参数值）

以下为书中实例：
Tester:

import org.jetbrains.annotations.NotNull;

public class ParamTest {
    public static void main(String[] args) {
        /*
         *Test1: Methods can't modify numeric parameters
         */
        System.out.println("Testing tripleValue:");
        double percent = 10;
        System.out.println("Before: percent=" + percent);
        tripValue(percent);
        System.out.println("After: percent=" + percent);
        /*
         *Test2: Methods can change the state of object parameters
         */
        System.out.println("\nTesting tripSalary:");
        var harry = new Employee("Harry", 5000);
        System.out.println("Before: salary=" + harry.getSalary());
        tripSalary(harry);
        System.out.println("After: salary=" + harry.getSalary());
        /*
         *Test3: Methods can't attach new objects to object parameters
         */
        System.out.println("\nTesting swap");
        var a = new Employee("Alice", 70000);
        var b = new Employee("Bob", 60000);
        System.out.println("Before: a="+a.getName());
        System.out.println("Before: b="+b.getName());
        swap(a, b);
        System.out.println("After: a=" + a.getName());
        System.out.println("After: b=" + b.getName());
    }

    public static void tripValue(double x) {
        x *= 3;
        System.out.println("End of method: x=" + x);
    }

    public static void tripSalary(@NotNull Employee x) {
        x.raiseSalary(200);
        System.out.println("End of method: salary=" + x.getSalary());
    }

    public static void swap(Employee x, Employee y) {
        Employee temp = x;
        x = y;
        y = temp;
        System.out.println("End of method: x=" + x.getName());
        System.out.println("End of method: y=" + y.getName());
    }
}

Employee:

public class Employee {
    private String name;
    private double salary;

    public Employee(String n, double s){
        name = n;
        salary = s;
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public void raiseSalary(double Percent) {
        double raise = salary * Percent / 100;
        salary += raise;
    }
}

输出：

Testing tripleValue:
Before: percent=10.0
End of method: x=30.0
After: percent=10.0

Testing tripSalary:
Before: salary=5000.0
End of method: salary=15000.0
After: salary=15000.0

Testing swap
Before: a=Alice
Before: b=Bob
End of method: x=Bob
End of method: y=Alice
After: a=Alice
After: b=Bob