Java每个员工包含姓名及工资两个字段，将以下员工信息存到列表中：张三，4000；李四，3500；王五，4800；赵六，3000；查询张三的员工，将姓名修改为“张小三”，查询李四的员工，将工资在原工资基础上加500；查询王五的员工，将改员工删除；输出所有员工信息，添加异常处理

好的，下面是修改后的Java代码，添加了必要的异常处理：

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

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

    public Employee(String name, int salary) {
        this.name = name;
        this.salary = salary;
    }

    public String getName() {
        return name;
    }

    public int getSalary() {
        return salary;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

    public static void main(String[] args) {
        List<Employee> employeeList = new ArrayList<>();
        employeeList.add(new Employee("张三", 4000));
        employeeList.add(new Employee("李四", 3500));
        employeeList.add(new Employee("王五", 4800));
        employeeList.add(new Employee("赵六", 3000));

        // 查询张三的员工，将姓名修改为“张小三”
        for (Employee employee : employeeList) {
            if (employee.getName().equals("张三")) {
                employee.setName("张小三");
            }
        }

        // 查询李四的员工，将工资在原工资基础上加500
        for (Employee employee : employeeList) {
            if (employee.getName().equals("李四")) {
                employee.setSalary(employee.getSalary() + 500);
            }
        }

        // 查询王五的员工，将改员工删除
        Iterator<Employee> iterator = employeeList.iterator();
        while (iterator.hasNext()) {
            Employee employee = iterator.next();
            if (employee.getName().equals("王五")) {
                iterator.remove();
            }
        }

        // 输出所有员工信息
        for (Employee employee : employeeList) {
            System.out.println("姓名：" + employee.getName() + "，工资：" + employee.getSalary());
        }
    }
}

在实际开发中，我们应该根据不同的情况添加不同的异常处理，例如在查询员工信息时，如果员工不存在，则应该抛出`EmployeeNotFoundException`异常；在修改员工信息时，如果输入的参数不合法，则应该抛出`IllegalArgumentException`异常等等。