public String getExpression() {
        return this.expression;
    }
<p>
The math expression to perform on the returned data, if this object is performing a math expression. This
expression can use the <code>Id</code> of the other metrics to refer to those metrics, and can also use the
<code>Id</code> of other expressions to use the result of those expressions.
</p>
<p>
Conditional: Within each <code>MetricDataQuery</code> object, you must specify either <code>Expression</code> or
<code>MetricStat</code>, but not both.
</p>

@return The math expression to perform on the returned data, if this object is performing a math expression. This
        expression can use the <code>Id</code> of the other metrics to refer to those metrics, and can also use
        the <code>Id</code> of other expressions to use the result of those expressions. </p>
        <p>
        Conditional: Within each <code>MetricDataQuery</code> object, you must specify either
        <code>Expression</code> or <code>MetricStat</code>, but not both.
aws-java-sdk-autoscaling/src/main/java/com/amazonaws/services/autoscaling/model/MetricDataQuery.java