为什么javax.servlet.jsp.JspContext是一个抽象类

Question

为什么这不是界面，我错过了什么？

public abstract class JspContext {
    public abstract void setAttribute(String var1, Object var2);
    public abstract void setAttribute(String var1, Object var2, int var3);
    public abstract Object getAttribute(String var1);
    public abstract Object getAttribute(String var1, int var2);
    public abstract Object findAttribute(String var1);
    public abstract void removeAttribute(String var1);
    public abstract void removeAttribute(String var1, int var2);
    public abstract int getAttributesScope(String var1);
    public abstract Enumeration<String> getAttributeNamesInScope(int var1);
    public abstract JspWriter getOut();
    public abstract ExpressionEvaluator getExpressionEvaluator();
    public abstract ELContext getELContext();
    public abstract VariableResolver getVariableResolver();

    public JspWriter pushBody(Writer writer) {
        return null;
    }

    public JspWriter popBody() {
        return null;
    }

}