public void setMultiplier(double multiplier) { // method head

    if (multiplier == 1.08 * 1.06 || multiplier == 1.08 || multiplier == 1.06) {
        this.multiplier = multiplier; // here the field variable is overwritten
    }

    throw new IllegalArgumentException("exception message");
}

您忘记编写类的字段变量。

设置器应覆盖字段变量。设置器应覆盖字段变量。他不返回任何东西，因此在方法头中为void而不是布尔值。如果参数错误，则抛出异常（尽可能有意义）。

提示：我不会在代码中的任何地方分发诸如1.06或1.08之类的常量。您可以将其定义如下：

public class student {

    private static final double MEANINGFUL_NAME_0 = 1.06;
    private static final double MEANINGFUL_NAME_1 = 1.08;

    // other code below
}

优点：如有必要，您仅需在一个位置更改常数。 您错过并写1.07而不是1.06的可能性更低。

setter应该更新一个值，并且不返回值（getter的角色），因此它的返回类型应该是void而不是boolean

public void setMultiplier(double multiplier) {
    if( multiplier == 1.08 * 1.06 || multiplier == 1.08 || multiplier == 1.06 ) {
        this.multiplier = multiplier; 
    }
}

我认为您只是忘了实际设置新值。

因此，如果您这样做，它应该可以工作：

void

此外，setter通常是this.setState((state,props) => { return { counter: state.counter + 1 } }); 而不是布尔值，并且没有返回值。

可能是常量的用例：

private static final double ONE_DOT_ZERO_SIX = 1.06f;
private static final double ONE_DOT_ZERO_EIGHT = 1.08f;

/**
 * Just to know if it is valid
 */
public boolean setMultiplier(double multiplier) {
    return (multiplier == ONE_DOT_ZERO_SIX || multiplier == ONE_DOT_ZERO_EIGHT
            || multiplier == (ONE_DOT_ZERO_SIX * ONE_DOT_ZERO_EIGHT));
}

/**
 * Return the multiplier if it is valid,otherwise 1
 */
public double setMultiplier(double multiplier) {
    if (multiplier == ONE_DOT_ZERO_SIX || multiplier == ONE_DOT_ZERO_EIGHT
            || multiplier == (ONE_DOT_ZERO_SIX * ONE_DOT_ZERO_EIGHT)) {
        return multiplier;
    } else {
        // return any invalid multiplier,this one doesn't change values at least
        return 1;   
    }
}

/**
 * Set the correct multiplier or a substitute
 */
public void setMultiplier(double multiplier) {
    if (multiplier == ONE_DOT_ZERO_SIX || multiplier == ONE_DOT_ZERO_EIGHT
            || multiplier == (ONE_DOT_ZERO_SIX * ONE_DOT_ZERO_EIGHT)) {
        this.multiplier = multiplier;
    } else {
        // return any invalid multiplier,this one doesn't change values at least
        this.multiplier = 1;
    }
}