我从您的问题中了解到的是，您需要将sum添加到patient object数组中特定对象的余额中。下面是方法，

（我排除了几个成员变量，这些变量不是通过查看您问题中的对象创建得到的，并且在patient类中只保留了name、patid和balance。我还假设您已经使用了所有字段的构造函数）

我把你的代码，并修改了你的要求一点。您可以参考我在代码段中添加的注解。
patientmainclass.class.类

public class PatientMainClass {

public static void main(String[] args) {
    Patient pAccount[] = new Patient[3];

    Patient p1 = new Patient("Jabari", "p01", 450);
    pAccount[0] = p1;
    Patient p2 = new Patient("Lisa", "p02", 300);
    pAccount[1] = p2;
    Patient p3 = new Patient("Kyle", "p03", 2000);
    pAccount[2] = p3;

    //Use bill method to add Amount to existing balance of all the Patient Objects
    Patient.bill(pAccount);
    System.out.println();
    for (int i = 0; i < pAccount.length; i++) {
        System.out.println("After adding amount to the Balance of pAccount[" + i + "] is : " + pAccount[i].getBalance());
    }

    System.out.println();
    //Use billToSpecific method to add Amount to specific Patient Object
    //to billToSpecific method, pass both Patient Array and Patient ID to which you want to add Amount
    Patient.billToSpecificPatient(pAccount, "p02");
    System.out.println();
    for (int i = 0; i < pAccount.length; i++) {
        System.out.println("After adding amount to p02, Balance of pAccount[" + i + "] is : " + pAccount[i].getBalance());
    }

} }

病人类别

public class Patient {

private String name;
private String patId;
private double balance;

public Patient(String name, String patId, double balance) {
    super();
    this.name = name;
    this.patId = patId;
    this.balance = balance;
}

public String getName() {
    return name;
}

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

public String getPatId() {
    return patId;
}

public void setPatId(String patId) {
    this.patId = patId;
}

public double getBalance() {
    return balance;
}

public void setBalance(double balance) {
    this.balance = balance;
}

// This method will Add entered value i.e.. "num" to the Balance of all the Patient Objects
public static void bill(Patient[] pAccount) {
    System.out.println("In bill method");           // Always try using System.out.println for Debugging
    Scanner in = new Scanner(System.in);
    double sum;

    double num = in.nextDouble();
    for (int i = 0; i < pAccount.length; i++) {
        Patient ad = pAccount[i];

        sum = ((Patient) ad).getBalance() + num;
        ad.setBalance(sum);
        System.out.println("Balance: " + sum);
        // break;                                   // Commenting break statement to add balance to all the Objects

    }

}

// This method will Add entered value i.e.. "num" to the Balance of the Specific Patient Object
public static void billToSpecificPatient(Patient[] pAccount, String patId) {
    System.out.println("In billToSpecific method"); // Always try using System.out.println for Debugging
    Scanner in = new Scanner(System.in);
    double sum;

    double num = in.nextDouble();
    for (int i = 0; i < pAccount.length; i++) {
        if (pAccount[i].getPatId().equals(patId)) {
            Patient ad = pAccount[i];
            sum = ((Patient) ad).getBalance() + num;
            ad.setBalance(sum);
            System.out.println("Balance: " + sum);
            break;                                  // Using break statement to exit the loop once we add amount to specific Patient Object
        }

    }

} }

我想，您现在可以通过这些代码片段来解决您的问题。希望对你有帮助。