Java method to add daily interest to bank account after month is over
Conventions

Java conventions state that code is indented with 4 spaces, not 2. Java also states that the brace should be on the same line, though this is more of your choice; the important part is consistency. After formatting, your code will then look like:

public void incrementDay() {
    if (!paidInFullFlag) {
        interestCharged += (this.balance + interestCharged) * (interestRate
                / 365);
    }
}

/*
 * Method adds interestCharged to balance and checks whether or not account
 * has been paid in full for month
 */
public void incrementMonth() {
    this.balance += interestCharged;
    this.interestCharged = 0;
    if (this.sumPayment &gt;= this.monthlyPayment) {
        this.paidInFullFlag = true;
    } else {
        this.paidInFullFlag = false;
    }

    if (minMonthly &lt; monthlyPayment &amp;&amp; sumPayment &lt; minMonthly) {
        this.balance += latePaymentPenalty;
    }
    this.sumPayment = 0;
    this.monthlyPayment = this.balance;
}


Miscellaneous

Here:

    if (this.sumPayment &gt;= this.monthlyPayment) {
        this.paidInFullFlag = true;
    } else {
        this.paidInFullFlag = false;
    }


Since the condition is a boolean already, this could be simplified to:

    this.paidInFullFlag = this.sumPayment &gt;= this.monthlyPayment;




The comment here:

/*
 * Method adds interestCharged to balance and checks whether or not account
 * has been paid in full for month
 */


Should be a JavaDoc comment:

/**
 * Method adds interestCharged to balance and checks whether or not account
 * has been paid in full for month
 */


Very similar, but not exactly the same.