在将changeToGive变成int而不是double（加上删除少量不必要的代码）之后，您将拥有一个工作解决方案：

int quarters = 0;
int dimes = 0;
int nickels = 0;
int pennies = 0;
Scanner sc = new Scanner(System.in);
int changeToGive = 0;

do {
    // if the user gave us a negative dollar amount
    if (changeToGive < 0)
        System.out.println("Dollar amounts must be positive.");

    // prompt the user for a monetary input
    System.out.println("How much change do you need to pay out?");

    // while the user's input is not a valid double, prompt again
    while (!sc.hasNextDouble()) {
        System.out.println("Invalid input. How much change do you need to pay out?");
        sc.next(); // this is important!
    }

    // if the user entered a valid double, assign it to changeToGive
    changeToGive = (int) (sc.nextDouble() * 100); // cents
} while (changeToGive <= 0); //if the user entered a valid double but it was negative, do it all again

while (changeToGive > 0) {
    if (changeToGive >= 25) {
        changeToGive -= 25;
        quarters++;
    } else if (changeToGive >= 10) {
        changeToGive -= 10;
        dimes++;
    } else if (changeToGive >= 5) {
        changeToGive -= 5;
        nickels++;
    } else {
        changeToGive -= 1;
        pennies++;
    }
}

System.out.print("Your change is:\n" + quarters + " quarters\n" + dimes + " dimes\n" + nickels + " nickels\n" + pennies + " pennies\n");

对于奖励标记，使用enum的硬币，每个硬币都有value字段，而不是硬编码。交给你去调查

• ".我不知道如何得到正确的结果打印出来，也不知道为什么它不给正确的React时，它是。..."*

您错误地将 5 cent 和 1 cent 值表示为 0.5 和 0.1，而不是 0.05 和 0.01。

if (changeToGive >= .5) changeToGive -= .5;
if (changeToGive >= .1) changeToGive -= .1;

此外，changeToGive 值在您扣除值时发生变化。
这是一个迭代 0.99。

0.99 - .25 = 0.74
0.74 - .25 = 0.49
0.49 - .25 = 0.24
0.24 - .10 = 0.13999999999999999
0.13999999999999999 - .10 = 0.03999999999999998
0.03999999999999998 - 0.01 = 0.029999999999999978
0.029999999999999978 - 0.01 = 0.019999999999999976
0.019999999999999976 - 0.01 = 0.009999999999999976

按照 Java 的建议，将 BigDecimal 类用于 currency 应用程序。

• Primitive Data Types (The Java™ Tutorials > Learning the Java Language > Language Basics)*。
• float数据类型.此数据类型不应用于精确值，例如货币。为此，您需要使用java.math.BigDecimal(https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/math/BigDecimal.html)类。*

这里有一个例子。

import static java.math.BigDecimal.ZERO;
import static java.math.RoundingMode.HALF_EVEN;

import java.math.BigDecimal;

int quarters = 0;
int dimes = 0;
int nickels = 0;
int pennies = 0;
//prompt the user for a monetary input
Scanner sc = new Scanner(System.in);
BigDecimal changeToGive = ZERO.setScale(2, HALF_EVEN), z = ZERO,
           q = new BigDecimal(".25"), d = new BigDecimal(".10"),
           n = new BigDecimal(".05"), p = new BigDecimal(".01");

do {
    //if the user gave us a negative dollar amount
    if(changeToGive.compareTo(z) < 0)
        System.out.println("Dollar amounts must be positive.");

        //if the current amount is zero dollars
    else if(changeToGive.compareTo(z) == 0)
        System.out.println("How much change do you need to pay out?");

    // while the user's input is not a valid double, prompt again
    while (!sc.hasNextBigDecimal()) {
        System.out.println("Invalid input.  How much change do you need to pay out?");
        sc.next(); // this is important!
    }

    //if the user entered a valid double, assign it to changeToGive
    changeToGive = sc.nextBigDecimal();
} while (changeToGive.compareTo(z) <= 0); //if the user entered a valid double but it was negative, do it all again

while (changeToGive.compareTo(z) > 0){
    if (changeToGive.compareTo(q) >= 0){
        changeToGive = changeToGive.subtract(q);
        quarters++;
    }else if (changeToGive.compareTo(d) >= 0){
        changeToGive = changeToGive.subtract(d);
        dimes++;
    }else if (changeToGive.compareTo(n) >= 0){
        changeToGive = changeToGive.subtract(n);
        nickels++;
    }else if (changeToGive.compareTo(p) >= 0){
        changeToGive = changeToGive.subtract(p);
        pennies++;
    }
}
System.out.print("Your change is:\n"+quarters+" quarters\n"+dimes+" dimes\n"+nickels+" nickels\n"+pennies+" pennies\n");

输出

How much change do you need to pay out?
.99
Your change is:
3 quarters
2 dimes
0 nickels
4 pennies